Update python versions

Currently support python 3.9-3.12 - which needs to be updated.

- Drop support for 3.9 (already EoL)
- Probably drop support for 3.10 (EoL this year - and PyPSA v1 requires >=3.11)
- Add 3.13 and 3.14
- (maybe optionally 3.14t - see if it works at least). 

Likely need to add tox/nox to implement / and locally.  Will also need to update CI/CD
