.. note::

   $ find sphinx tests LICENSE doc/conf.py -type f -exec sed -i '' -e 's/2007\-20../2007-2021/' {} \;
   $ git co sphinx/locale/**/*.js sphinx/templates/epub3/mimetype

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.

At once, we added $SPHINX_TEST_TEMPDIR to keep intermediate files of
testing to investigate the failures.  At present, we've used pytest
as a test runner.  And it keeps temporary directories of last 3
testings.  So this starts to use it instead of $SPHINX_TEST_TEMPDIR.
https://docs.pytest.org/en/latest/tmpdir.html#the-default-base-temporary-directory

commits are squashed.

* Add PathComparer for testing and fix 2 test failure on Windows.
* fix flake8
* add type information by f2f review. Thanks to @tk0miya!
* fix mypy, flake8 again..