Drop Python 3.9

[anaprietonem]

What maintenance does this project need?

Python 3.9 is reaching its End-of-Life (EOL) in October this year. Additionally, our CI workflows are encountering increasing difficulties with Python 3.9, particularly regarding type hinting incompatibilities, such as issues related to UP007.

Given these challenges and the fact that Python 3.9 will soon be unsupported, I propose removing Python 3.9 support from our CI pipelines and updating our documentation accordingly.

Proposed Changes:
Remove Python 3.9 from CI workflows.
Update dependencies and type hints where necessary.
Adjust documentation to reflect the new minimum supported Python version.

Organisation

ECMWF