Black rolls out style changes every year, and using "stable" means that
the check run on PRs might start formatting differently than we do
locally, or require a reformat of the codebase to make a PR submittable.
Pin to the version that we've been using. We should update to 23 at some
point, but we want to do that deliberately.
This is just the first step to prove the concept. Most of the work done
in our workflows can be split into separate jobs to parallelize the
workflow. This will also make the checks page more readable.
This change alone probably won't speed up CI much.
