ci: move black & isort to pre-commit

[browniebroke]

pre-commit is great tool to run these kind of auto-formatters in isolated environments. There are more hooks which are supported, but I haven't introduced any new ones for now. I just moved 2 packages that were already used here, and use the GitHub action to run them on CI.

The step further is to use pre-commit.ci which has a couple of extra features:

• auto-fixes PRs where possible
• keep hooks up to date
• does very aggressive caching, which means it runs super fast

However, this requires a maintainer to sign-up, and I appreciate that it's yet another service.