https://github.com/asottile/pyupgrade

> A tool to automatically upgrade syntax for newer versions of the
> language.

- Drop u str prefix
- Drop base object inheritance
- Drop args to super()
- Use set literals
- Use dict comprehension
- Use set comprehension

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

Since python3.5, subprocess.run() has been introduced. It works a
wrapper of Popen, and it looks much simple and better. This uses it
instead of Popen to make our code simple.

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..