[lang-kit] Update Python to `python-3.10`

Python version `3.12` brought performance improvements that now are being used by the pulsar astrophysics community. The NANOGrav collaboration, for example, has standardized on it. It would be good for us to update, as the world is moving forward and our `python-3.9` is getting stale.

This issue names the lang-kit area but no file or test. First locate where the current python-3.9 version is configured and confirm whether the requested target is python-3.10 or python-3.12. Done means the selected Python version is updated and the relevant checks pass.

Written by the indexing model from the issue text.