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_build_html.py

@@ -11,7 +11,7 @@
 import os
 import re
 from distutils.version import LooseVersion
-from itertools import cycle, chain
+from itertools import chain, cycle
 
 import pygments
 import pytest
@@ -23,7 +23,6 @@ from sphinx.testing.util import strip_escseq
 from sphinx.util import docutils, md5
 from sphinx.util.inventory import InventoryFile
 
-
 ENV_WARNINGS = """\
 %(root)s/autodoc_fodder.py:docstring of autodoc_fodder.MarkupError:\\d+: \
 WARNING: Explicit markup ends without a blank line; unexpected unindent.