Check Python code formatting in CI

[guitargeek]

Explain what you would like to see improved and how.

People regularly ask how the Python code contributed to ROOT should be formatted, or they submit code that needs to be re-formatted during the PR review.

It would be better to have a well-defined style and a CI check that enforces it.

Most Python code in the repository is formatted with black, using only the --line-length=120 option to be consistent with our C++ code style.

Maybe one could implement such a check using the GitHub Actions integration of black.

[andresailer]

Alternatively ruff now has the possibility to do the same formatting as black.

https://docs.astral.sh/ruff/formatter/