Sort imports with isort

Keep imports alphabetically sorted and their order homogeneous across
Python source files.

The isort project has more feature and is more active than the
flake8-import-order plugin.

Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.

tests/test_ext_napoleon.py

@@ -14,8 +14,8 @@ from collections import namedtuple
 from unittest import TestCase, mock
 
 from sphinx.application import Sphinx
+from sphinx.ext.napoleon import Config, _process_docstring, _skip_member, setup
 from sphinx.testing.util import simple_decorator
-from sphinx.ext.napoleon import _process_docstring, _skip_member, Config, setup
 
 
 def _private_doc():