Update python versions used in github actions

[maarten-ic]

• Remove Python 3.8 (which is already end-of-life for ~10 months) and add Python 3.13.
• Update OS to ubuntu-latest instead of a fixed version

[SimonPinches]

Despite 3.8 being end-of-life, I'd still maintain the testing for it (and 3.7) since I'm not sure we can guarantee all our collaborators have moved on and according to our pyproject.toml file, we're still compatible with `>=3.7". Or is this now not possible using GitHub actions?

[maarten-ic]

Despite 3.8 being end-of-life, I'd still maintain the testing for it (and 3.7) since I'm not sure we can guarantee all our collaborators have moved on and according to our pyproject.toml file, we're still compatible with `>=3.7". Or is this now not possible using GitHub actions?

It's (at this moment) still possible to test with Python 3.8 on github actions. For Python 3.7 I'll test if that works.

For MUSCLE3, the biggest problem with testing for outdated Python versions was that, at some point, dependencies were no longer available on PyPI for the older Python version. We can just choose to disable tests for old python versions when this becomes an issue.

[maarten-ic]

Update: Python 3.7 cannot be tested as is on github actions, because the build dependency (setuptools_scm>8) is not available on PyPI for Python 3.7.

[SimonPinches]

lgtm