Drop Python 3.9 support?

[ElliottKasoar]

This has been discussed at various stages (e.g. #303, #312, #328, #372), but at some point we will drop support for Python 3.9.

My general feeling was to roughly stick to the Python release cycle, or at least wait until we can properly support 3.13, but some packages (roughly) follow NEP29, which has already dropped 3.9.

In particular, this seems to be the case for Materials Project packages e.g. pymatgen, and projects which interact with it e.g. CHGNet.

It's worth noting that PyTorch decided against dropping 3.9 yet, but perhaps more practically, the question is how much we lose from dropping support now? My impression is that most HPC users are required to to install a new version of Python anyway, and I would imagine locally using a different version is less of an issue.

The issue discussed in #347 may also be fixable.