Migrate to more recent Python versions

[pirmink]

The code is currently using Python 3.10. In the meantime, the MeteoSwiss development tools support also 3.11, 3.12, and 3.13. 3.10 is soon to be phased out. pillow was the limiting package that generated warnings when run with 3.11.

• Migrate code to Python 3.11. Find a solution to use the pillow package (or a replacement) without warning messages. Solve other future and obsolescence warnings and fully test the resulting code
• If successful, migrate to 3.12 and solve future and obsolescence warnings, fully test resulting code
• If successful, migrate to 3.13 and solve future and obsolescence warnings, fully test resulting code

Set the desired Python version in pyproject.toml and create an environment with Poetry without using the poetry.lock file. Poetry stores the used version in the file .python-version, check if this has been updated after creating the environment.