ci: Move black to pre-commit

[auscompgeek]

This intends to do two things:

1. Pin the black version, so we don't get surprised by the annual black major release suddenly failing CI.
2. Make it easier for first-time contributors to discover the formatters/linters we have, and make these checks easier to reproduce locally (e.g. pre-commit run --all-files).

Once this is in we can move the pre-commit job off of GitHub Actions (to avoid hitting concurrent runner limits) by using https://pre-commit.ci instead (which will also provide autoupdate PRs and autofixing).