Drop support for Python 3.9 and begin supporting 3.12

[ma-sadeghi]

This is a bit opinionated so I don't consider it as a show stopper, but I highly encourage you to consider it.

cellpy depends on NumPy, and they will drop support for Python 3.9 in a month (Apr 4, 2024).

[jepegit]

Adding support for Python 3.12 requires some work on the OtherPath object since it builds on pathlib that had a major upgrade going from Python 3.11 to Python 3.12. Should be implemented before v1.1.

[jepegit]

Closing it for now - will reopen it soon when we start making the necessary changes in OtherPaths for supporting py3.12

[ma-sadeghi]

Let's keep this open since it's due soon, but I'll approve it for JOSS review.