Add cpython & other repos as formatting stability check to CI

[konstin]

We have a script that formats cpython twice and checks that the formatting stays the same, that we only output valid syntax and that we don't panic due to some input. I want to integrate this script into CI to avoid regressions. This also needs documentation on how to run the check locally.

[cnpryer]

This also needs documentation on how to run the check locally

Is this already solved or is there more that could be helpful to document?

[konstin]

The general way is documented at https://github.com/astral-sh/ruff/blob/main/crates/ruff_python_formatter/README.md#checking-entire-projects. I'm currently working a small but diverse set of projects that we continuously check and will add more instructions once that's done.

I also ran the stability checks with the 3k repo from ruff-usage-aggregate and have to go through all of them and either fix them directly or open issues for each distinct one.

[konstin]

i've posted the initial script in #5828, this ticket is effectively about automating most of the scores from that ticket.

[konstin]

Fixed by #5919