Infra: update linting/formatting for non-peps/ dirs

[hugovk]

I find it confusing we run Black on some dirs, and the Ruff formatter on others, and no formatter on others.

Similarly, we Ruff lint some dirs, and not others.

Let's pick just one formatter, and apply to all directories (except peps/). I picked Black, but don't mind using Ruff instead.

And let's also lint all dirs (except peps/).

Finally, combine the two .ruff.toml files into a top-level pyproject.toml, and add a couple of lint rules.