Replace black with ruff in `tidy`
`ruff` can both lint and format Python code (in fact, it should be a mostly drop-in replacement for `black` in terms of formatting), so it's not needed to use `black` anymore. This PR removes `black` and replaces it with `ruff`, to get rid of one Python dependency, and also to make Python formatting faster (although that's a small thing).
If we decide to merge this, we'll need to "reformat the world" - `ruff` is not perfectly compatible with `black`, and it also looks like `black` was actually ignoring some files before. I tried it locally (`./x test tidy --extra-checks=py:fmt --bless`) and it also reformatted some code in subtrees (e.g. `clippy` or `rustc_codegen_gcc`) - I'm not sure how to handle that.
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