Minimum supported python version

[agoscinski]

I think we should stick with python 3.9 as minimum python version as a lot of software packages we rely on also supports it and so far I only see quality of life features as a reason to stick with higher versions. But we can enforce this in the CI, when we come closer to release, since then python 3.9 will be end of life and see if other packages update their version.

[agoscinski]

We did the opposite and fixed to Python 3.12 in #240 for now to be consistent with aiida-icon and speed up development by the quality of life features. A downgrade to Python would mainly mean so far to change some typehints, that can be done later.