.. 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.

In development of 3.0, Sphinx starts to obey to the rule of
"Identifier Normalization" of docutils.  This extends it to allow
dots(".") and underscores("_") for node identifier.

It allows Sphinx to generate node identifier from source string as
possible as it is (bacause dots and underscores are usually used in
many programming langauges).

This change will keep not to break hyperlinks as possible.

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

Note: all warnings are tested on test-warnings.