diff --git a/sphinx/builders/epub3.py b/sphinx/builders/epub3.py
index c1e6ab98a..8e3028bf5 100644
--- a/sphinx/builders/epub3.py
+++ b/sphinx/builders/epub3.py
@@ -13,8 +13,6 @@
 from collections import namedtuple
 from os import path
 
-from six import string_types
-
 from sphinx import package_dir
 from sphinx.builders import _epub_base
 from sphinx.config import string_classes, ENUM
@@ -234,7 +232,7 @@ def convert_epub_css_files(app, config):
     """This converts string styled epub_css_files to tuple styled one."""
     epub_css_files = []  # type: List[Tuple[unicode, Dict]]
     for entry in config.epub_css_files:
-        if isinstance(entry, string_types):
+        if isinstance(entry, str):
             epub_css_files.append((entry, {}))
         else:
             try:
diff --git a/sphinx/builders/html.py b/sphinx/builders/html.py
index 34513faa5..07a44b2ad 100644
--- a/sphinx/builders/html.py
+++ b/sphinx/builders/html.py
@@ -25,7 +25,7 @@ from docutils.frontend import OptionParser
 from docutils.io import DocTreeInput, StringOutput
 from docutils.readers.doctree import Reader as DoctreeReader
 from docutils.utils import relative_path
-from six import text_type, string_types
+from six import text_type
 
 from sphinx import package_dir, __display_version__
 from sphinx.application import ENV_PICKLE_FILENAME
@@ -531,7 +531,7 @@ class StandaloneHTMLBuilder(Builder):
         favicon = self.config.html_favicon and \
             path.basename(self.config.html_favicon) or ''
 
-        if not isinstance(self.config.html_use_opensearch, string_types):
+        if not isinstance(self.config.html_use_opensearch, str):
             logger.warning(__('html_use_opensearch config value must now be a string'))
 
         self.relations = self.env.collect_relations()
@@ -1533,7 +1533,7 @@ def convert_html_css_files(app, config):
     """This converts string styled html_css_files to tuple styled one."""
     html_css_files = []  # type: List[Tuple[unicode, Dict]]
     for entry in config.html_css_files:
-        if isinstance(entry, string_types):
+        if isinstance(entry, str):
             html_css_files.append((entry, {}))
         else:
             try:
@@ -1551,7 +1551,7 @@ def convert_html_js_files(app, config):
     """This converts string styled html_js_files to tuple styled one."""
     html_js_files = []  # type: List[Tuple[unicode, Dict]]
     for entry in config.html_js_files:
-        if isinstance(entry, string_types):
+        if isinstance(entry, str):
             html_js_files.append((entry, {}))
         else:
             try:
diff --git a/sphinx/builders/manpage.py b/sphinx/builders/manpage.py
index 25287f2be..a7bbe782d 100644
--- a/sphinx/builders/manpage.py
+++ b/sphinx/builders/manpage.py
@@ -13,7 +13,6 @@ from os import path
 
 from docutils.frontend import OptionParser
 from docutils.io import FileOutput
-from six import string_types
 
 from sphinx import addnodes
 from sphinx.builders import Builder
@@ -77,7 +76,7 @@ class ManualPageBuilder(Builder):
                 logger.warning(__('"man_pages" config value references unknown '
                                   'document %s'), docname)
                 continue
-            if isinstance(authors, string_types):
+            if isinstance(authors, str):
                 if authors:
                     authors = [authors]
                 else:
diff --git a/sphinx/config.py b/sphinx/config.py
index 2509e0b1b..e12889c61 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -17,7 +17,7 @@ from collections import OrderedDict
 from os import path, getenv
 from typing import Any, NamedTuple, Union
 
-from six import string_types, text_type, integer_types
+from six import text_type, integer_types
 
 from sphinx.deprecation import RemovedInSphinx30Warning, RemovedInSphinx40Warning
 from sphinx.errors import ConfigError, ExtensionError
@@ -183,7 +183,7 @@ class Config:
         self.setup = config.get('setup', None)  # type: Callable
 
         if 'extensions' in overrides:
-            if isinstance(overrides['extensions'], string_types):
+            if isinstance(overrides['extensions'], str):
                 config['extensions'] = overrides.pop('extensions').split(',')
             else:
                 config['extensions'] = overrides.pop('extensions')
@@ -211,7 +211,7 @@ class Config:
 
     def convert_overrides(self, name, value):
         # type: (unicode, Any) -> Any
-        if not isinstance(value, string_types):
+        if not isinstance(value, str):
             return value
         else:
             defvalue = self.values[name][0]
@@ -231,7 +231,7 @@ class Config:
                                      (value, name))
             elif hasattr(defvalue, '__call__'):
                 return value
-            elif defvalue is not None and not isinstance(defvalue, string_types):
+            elif defvalue is not None and not isinstance(defvalue, str):
                 raise ValueError(__('cannot override config setting %r with unsupported '
                                     'type, ignoring') % name)
             else:
@@ -265,7 +265,7 @@ class Config:
                     logger.warning(__('unknown config value %r in override, ignoring'),
                                    valname)
                     continue
-                if isinstance(value, string_types):
+                if isinstance(value, str):
                     config[valname] = self.convert_overrides(valname, value)
                 else:
                     config[valname] = value
@@ -316,7 +316,7 @@ class Config:
 
     def filter(self, rebuild):
         # type: (Union[unicode, List[unicode]]) -> Iterator[ConfigValue]
-        if isinstance(rebuild, string_types):
+        if isinstance(rebuild, str):
             rebuild = [rebuild]
         return (value for value in self if value.rebuild in rebuild)
 
@@ -383,7 +383,7 @@ def convert_source_suffix(app, config):
     * new style: a dict which maps from fileext to filetype
     """
     source_suffix = config.source_suffix
-    if isinstance(source_suffix, string_types):
+    if isinstance(source_suffix, str):
         # if str, considers as default filetype (None)
         #
         # The default filetype is determined on later step.
diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index 580478c21..2fd388c8f 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -18,7 +18,7 @@ import warnings
 from typing import Any
 
 from docutils.statemachine import ViewList
-from six import text_type, string_types
+from six import text_type
 
 import sphinx
 from sphinx.deprecation import RemovedInSphinx30Warning, RemovedInSphinx40Warning
@@ -836,7 +836,7 @@ class ModuleDocumenter(Documenter):
                 memberlist = self.object.__all__
                 # Sometimes __all__ is broken...
                 if not isinstance(memberlist, (list, tuple)) or not \
-                   all(isinstance(entry, string_types) for entry in memberlist):
+                   all(isinstance(entry, str) for entry in memberlist):
                     logger.warning(
                         __('__all__ should be a list of strings, not %r '
                            '(in module %s) -- ignoring __all__') %
@@ -1460,7 +1460,7 @@ def merge_autodoc_default_flags(app, config):
                   RemovedInSphinx30Warning, stacklevel=2)
 
     for option in config.autodoc_default_flags:
-        if isinstance(option, string_types):
+        if isinstance(option, str):
             config.autodoc_default_options[option] = None
         else:
             logger.warning(
diff --git a/sphinx/ext/autosummary/__init__.py b/sphinx/ext/autosummary/__init__.py
index 10adfc9ce..f49725f39 100644
--- a/sphinx/ext/autosummary/__init__.py
+++ b/sphinx/ext/autosummary/__init__.py
@@ -64,7 +64,6 @@ from docutils import nodes
 from docutils.parsers.rst import directives
 from docutils.parsers.rst.states import RSTStateMachine, state_classes
 from docutils.statemachine import ViewList
-from six import string_types
 from six import text_type
 
 import sphinx
@@ -642,8 +641,8 @@ def get_rst_suffix(app):
         parser_class = app.registry.get_source_parsers().get(suffix)
         if parser_class is None:
             return ('restructuredtext',)
-        if isinstance(parser_class, string_types):
-            parser_class = import_object(parser_class, 'source parser')  # type: ignore
+        if isinstance(parser_class, str):
+            parser_class = import_object(parser_class, 'source parser')
         return parser_class.supported
 
     suffix = None  # type: unicode
diff --git a/sphinx/ext/intersphinx.py b/sphinx/ext/intersphinx.py
index 5a16f21d3..e144adf84 100644
--- a/sphinx/ext/intersphinx.py
+++ b/sphinx/ext/intersphinx.py
@@ -35,7 +35,7 @@ from urllib.parse import urlsplit, urlunsplit
 
 from docutils import nodes
 from docutils.utils import relative_path
-from six import string_types, text_type
+from six import text_type
 
 import sphinx
 from sphinx.builders.html import INVENTORY_FILENAME
@@ -217,7 +217,7 @@ def load_mappings(app):
         if isinstance(value, (list, tuple)):
             # new format
             name, (uri, inv) = key, value
-            if not isinstance(name, string_types):
+            if not isinstance(name, str):
                 logger.warning(__('intersphinx identifier %r is not string. Ignored'), name)
                 continue
         else:
diff --git a/sphinx/ext/napoleon/docstring.py b/sphinx/ext/napoleon/docstring.py
index 401193b1d..9841f5462 100644
--- a/sphinx/ext/napoleon/docstring.py
+++ b/sphinx/ext/napoleon/docstring.py
@@ -16,8 +16,6 @@ import re
 from collections.abc import Callable
 from functools import partial
 
-from six import string_types
-
 from sphinx.ext.napoleon.iterators import modify_iter
 from sphinx.locale import _
 from sphinx.util.pycompat import UnicodeMixin
@@ -132,10 +130,11 @@ class GoogleDocstring(UnicodeMixin):
         self._name = name
         self._obj = obj
         self._opt = options
-        if isinstance(docstring, string_types):
-            docstring = docstring.splitlines()
-        self._lines = docstring
-        self._line_iter = modify_iter(docstring, modifier=lambda s: s.rstrip())
+        if isinstance(docstring, str):
+            lines = docstring.splitlines()
+        else:
+            lines = docstring  # type: ignore
+        self._line_iter = modify_iter(lines, modifier=lambda s: s.rstrip())
         self._parsed_lines = []  # type: List[unicode]
         self._is_in_section = False
         self._section_indent = 0
@@ -543,7 +542,7 @@ class GoogleDocstring(UnicodeMixin):
 
         if self._config.napoleon_custom_sections is not None:
             for entry in self._config.napoleon_custom_sections:
-                if isinstance(entry, string_types):
+                if isinstance(entry, str):
                     # if entry is just a label, add to sections list,
                     # using generic section logic.
                     self._sections[entry.lower()] = self._parse_custom_generic_section
@@ -961,7 +960,7 @@ class NumpyDocstring(GoogleDocstring):
         # type: () -> bool
         section, underline = self._line_iter.peek(2)
         section = section.lower()
-        if section in self._sections and isinstance(underline, string_types):
+        if section in self._sections and isinstance(underline, str):
             return bool(_numpy_section_regex.match(underline))
         elif self._directive_sections:
             if _directive_regex.match(section):
diff --git a/sphinx/jinja2glue.py b/sphinx/jinja2glue.py
index c3ed86e09..739437a16 100644
--- a/sphinx/jinja2glue.py
+++ b/sphinx/jinja2glue.py
@@ -17,7 +17,6 @@ from jinja2 import FileSystemLoader, BaseLoader, TemplateNotFound, \
     contextfunction
 from jinja2.sandbox import SandboxedEnvironment
 from jinja2.utils import open_if_exists
-from six import string_types
 
 from sphinx.application import TemplateBridge
 from sphinx.util import logging
@@ -33,7 +32,7 @@ if False:
 
 def _tobool(val):
     # type: (unicode) -> bool
-    if isinstance(val, string_types):
+    if isinstance(val, str):
         return val.lower() in ('true', '1', 'yes', 'on')
     return bool(val)
 
diff --git a/sphinx/search/__init__.py b/sphinx/search/__init__.py
index 3008ab6f3..f92120010 100644
--- a/sphinx/search/__init__.py
+++ b/sphinx/search/__init__.py
@@ -12,7 +12,7 @@ import pickle
 import re
 from os import path
 
-from six import text_type, string_types
+from six import text_type
 
 from docutils.nodes import raw, comment, title, Text, NodeVisitor, SkipNode
 
@@ -291,7 +291,7 @@ class IndexBuilder:
     def load(self, stream, format):
         # type: (IO, Any) -> None
         """Reconstruct from frozen data."""
-        if isinstance(format, string_types):
+        if isinstance(format, str):
             format = self.formats[format]
         frozen = format.load(stream)
         # if an old index is present, we treat it as not existing.
@@ -319,7 +319,7 @@ class IndexBuilder:
     def dump(self, stream, format):
         # type: (IO, Any) -> None
         """Dump the frozen index to a stream."""
-        if isinstance(format, string_types):
+        if isinstance(format, str):
             format = self.formats[format]
         format.dump(self.freeze(), stream)
 
diff --git a/sphinx/setup_command.py b/sphinx/setup_command.py
index 733bc4e34..072af6c91 100644
--- a/sphinx/setup_command.py
+++ b/sphinx/setup_command.py
@@ -18,7 +18,7 @@ import sys
 from distutils.cmd import Command
 from distutils.errors import DistutilsOptionError, DistutilsExecError
 
-from six import StringIO, string_types
+from six import StringIO
 
 from sphinx.application import Sphinx
 from sphinx.cmd.build import handle_exception
@@ -129,7 +129,7 @@ class BuildDoc(Command):
         if val is None:
             setattr(self, option, default)
             return default
-        elif not isinstance(val, string_types):
+        elif not isinstance(val, str):
             raise DistutilsOptionError("'%s' must be a %s (got `%s`)"
                                        % (option, what, val))
         return val
diff --git a/sphinx/testing/fixtures.py b/sphinx/testing/fixtures.py
index 4ea634b10..968807788 100644
--- a/sphinx/testing/fixtures.py
+++ b/sphinx/testing/fixtures.py
@@ -17,7 +17,7 @@ from collections import namedtuple
 from tempfile import gettempdir
 
 import pytest
-from six import StringIO, string_types
+from six import StringIO
 
 from . import util
 
@@ -102,8 +102,7 @@ def test_params(request):
     }
     result.update(kwargs)
 
-    if (result['shared_result'] and
-            not isinstance(result['shared_result'], string_types)):
+    if (result['shared_result'] and not isinstance(result['shared_result'], str)):
         raise pytest.Exception('You can only provide a string type of value '
                                'for "shared_result" ')
     return result
diff --git a/sphinx/testing/util.py b/sphinx/testing/util.py
index 1f575569a..cb2d9da4a 100644
--- a/sphinx/testing/util.py
+++ b/sphinx/testing/util.py
@@ -16,7 +16,6 @@ from xml.etree import ElementTree
 
 from docutils import nodes
 from docutils.parsers.rst import directives, roles
-from six import string_types
 
 from sphinx import application, locale
 from sphinx.builders.latex import LaTeXBuilder
@@ -74,7 +73,7 @@ def assert_node(node, cls=None, xpath="", **kwargs):
             for i, nodecls in enumerate(cls):
                 path = xpath + "[%d]" % i
                 assert_node(node[i], nodecls, xpath=path, **kwargs)
-        elif isinstance(cls, string_types):
+        elif isinstance(cls, str):
             assert node == cls, 'The node %r is not %r: %r' % (xpath, cls, node)
         else:
             assert isinstance(node, cls), \
diff --git a/sphinx/util/compat.py b/sphinx/util/compat.py
index 1f3b0f445..e0c86df5e 100644
--- a/sphinx/util/compat.py
+++ b/sphinx/util/compat.py
@@ -15,7 +15,6 @@ import sys
 import warnings
 
 from docutils.utils import get_source_line
-from six import string_types
 
 from sphinx import addnodes
 from sphinx.deprecation import RemovedInSphinx30Warning, RemovedInSphinx40Warning
@@ -36,8 +35,8 @@ def deprecate_source_parsers(app, config):
                       'Please use app.add_source_parser() API instead.',
                       RemovedInSphinx30Warning)
         for suffix, parser in config.source_parsers.items():
-            if isinstance(parser, string_types):
-                parser = import_object(parser, 'source parser')  # type: ignore
+            if isinstance(parser, str):
+                parser = import_object(parser, 'source parser')
             app.add_source_parser(suffix, parser)
 
 
diff --git a/sphinx/util/inspect.py b/sphinx/util/inspect.py
index 885a7b929..5f2bfe461 100644
--- a/sphinx/util/inspect.py
+++ b/sphinx/util/inspect.py
@@ -18,7 +18,7 @@ import sys
 import typing
 from functools import partial
 
-from six import StringIO, string_types
+from six import StringIO
 
 from sphinx.util import logging
 from sphinx.util.pycompat import NoneType
@@ -381,8 +381,7 @@ class Signature:
                               param.KEYWORD_ONLY):
                 arg.write(param.name)
                 if param.annotation is not param.empty:
-                    if isinstance(param.annotation, string_types) and \
-                            param.name in self.annotations:
+                    if isinstance(param.annotation, str) and param.name in self.annotations:
                         arg.write(': ')
                         arg.write(self.format_annotation(self.annotations[param.name]))
                     else:
@@ -424,8 +423,8 @@ class Signature:
 
         Displaying complex types from ``typing`` relies on its private API.
         """
-        if isinstance(annotation, string_types):
-            return annotation  # type: ignore
+        if isinstance(annotation, str):
+            return annotation
         elif isinstance(annotation, typing.TypeVar):  # type: ignore
             return annotation.__name__
         elif not annotation:
diff --git a/sphinx/util/jsdump.py b/sphinx/util/jsdump.py
index acdeb3083..e7b1146a2 100644
--- a/sphinx/util/jsdump.py
+++ b/sphinx/util/jsdump.py
@@ -12,7 +12,7 @@
 
 import re
 
-from six import integer_types, string_types
+from six import integer_types
 
 if False:
     # For type annotation
@@ -84,7 +84,7 @@ double   in   super""".split())
 def dumps(obj, key=False):
     # type: (Any, bool) -> str
     if key:
-        if not isinstance(obj, string_types):
+        if not isinstance(obj, str):
             obj = str(obj)
         if _nameonly_re.match(obj) and obj not in reswords:
             return obj  # return it as a bare word
@@ -105,8 +105,8 @@ def dumps(obj, key=False):
         return '[%s]' % ','.join(sorted(dumps(x) for x in obj))
     elif isinstance(obj, (tuple, list)):
         return '[%s]' % ','.join(dumps(x) for x in obj)
-    elif isinstance(obj, string_types):
-        return encode_string(obj)  # type: ignore
+    elif isinstance(obj, str):
+        return encode_string(obj)
     raise TypeError(type(obj))
 
 
diff --git a/sphinx/util/requests.py b/sphinx/util/requests.py
index a2bb22fe8..020cad08b 100644
--- a/sphinx/util/requests.py
+++ b/sphinx/util/requests.py
@@ -17,7 +17,6 @@ from urllib.parse import urlsplit
 
 import pkg_resources
 import requests
-from six import string_types
 
 try:
     from requests.packages.urllib3.exceptions import SSLError
@@ -120,7 +119,7 @@ def _get_tls_cacert(url, config):
     certs = getattr(config, 'tls_cacerts', None)
     if not certs:
         return True
-    elif isinstance(certs, (string_types, tuple)):
+    elif isinstance(certs, (str, tuple)):
         return certs  # type: ignore
     else:
         hostname = urlsplit(url)[1]
diff --git a/sphinx/writers/html.py b/sphinx/writers/html.py
index fb7c26baf..c296bc884 100644
--- a/sphinx/writers/html.py
+++ b/sphinx/writers/html.py
@@ -17,7 +17,6 @@ import warnings
 
 from docutils import nodes
 from docutils.writers.html4css1 import Writer, HTMLTranslator as BaseTranslator
-from six import string_types
 
 from sphinx import addnodes
 from sphinx.deprecation import RemovedInSphinx30Warning
@@ -81,7 +80,7 @@ class HTMLTranslator(BaseTranslator):
         self.protect_literal_text = 0
         self.permalink_text = builder.config.html_add_permalinks
         # support backwards-compatible setting to a bool
-        if not isinstance(self.permalink_text, string_types):
+        if not isinstance(self.permalink_text, str):
             self.permalink_text = self.permalink_text and u'\u00B6' or ''
         self.permalink_text = self.encode(self.permalink_text)
         self.secnumber_suffix = builder.config.html_secnumber_suffix
diff --git a/sphinx/writers/html5.py b/sphinx/writers/html5.py
index 4a80c0cbb..70154923e 100644
--- a/sphinx/writers/html5.py
+++ b/sphinx/writers/html5.py
@@ -16,7 +16,6 @@ import warnings
 
 from docutils import nodes
 from docutils.writers.html5_polyglot import HTMLTranslator as BaseTranslator
-from six import string_types
 
 from sphinx import addnodes
 from sphinx.deprecation import RemovedInSphinx30Warning
@@ -51,7 +50,7 @@ class HTML5Translator(BaseTranslator):
         self.protect_literal_text = 0
         self.permalink_text = builder.config.html_add_permalinks
         # support backwards-compatible setting to a bool
-        if not isinstance(self.permalink_text, string_types):
+        if not isinstance(self.permalink_text, str):
             self.permalink_text = self.permalink_text and u'\u00B6' or ''
         self.permalink_text = self.encode(self.permalink_text)
         self.secnumber_suffix = builder.config.html_secnumber_suffix