FR: Establish a default python formatter / linter and enforce it via CI

Note: Originally copied / imported from https://github.com/web-platform-tests/wpt/issues/42710. We'll keep the other bug open to track the implementation side of this FR.

So long as we do not agree on a default formatter for this repository, back-end-forth diff noise and bikeshedding keeps coming up (e.g.).

It would be the best to everyone, and for improved devex, to agree to use one and only one formatter.
Additionally, to ensure it sticks, we should add some CI job to enforce that no violations are introduced.

The status quo (at least within the realm of WebDriver BiDi) seems to be the following:

• Google / Chromium and the Chromium BiDi Mapper use yapf v0.31.0.
• Mozilla / Firefox uses black.

The following AIs should happen:

1. agree on which formatter to use: yapf, black, or something else?
2. globally format the codebase using it
3. add a (non-optional?) CI check to enforce it

Resources:

• yapf: https://github.com/google/yapf
• black: https://github.com/psf/black

cc @juliandescottes @past

Start by reviewing the linked WPT issue, the referenced pull-request discussion, and the yapf and black resources. Done means the project agrees on one formatter, the codebase is formatted with it, and CI enforces that new violations are not introduced.