Migrate to more recent Python versions

[pirmink]

The code is currently using Python 3.11. In the meantime, the MeteoSwiss development tools support also 3.12 and 3.13.

• Migrate code to Python 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 requirements/requirements.txt and create an unpinned Conda environment with this file instead of using the pinned dependencies in environment.yml.