.. 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.

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

Split testing of changes builder to its own test file and root.
Improve coverage a little, and add case that fails if a module
directive is included in the rst source.

Correctly handle changesets with a module declared.

Fixes https://github.com/sphinx-doc/sphinx/issues/6134