Fix python version in tests

[SimonKamuk]

Describe your changes

Tests did not use the pinned python version from the test matrix, they all ran on newest available version (3.12).
Therefore, a bug caused by using the typing annotation with | instead of Union did not raise an error, even though that is only compatible with python >=3.10.

This PR fixes that bug, and lets the tests use the expected python version.

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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[SimonKamuk]

Looks like xarray>=2024.2.0 requires python 3.9 or later. Should we drop support for 3.8? @khintz @leifdenby

[khintz]

Looks like xarray>=2024.2.0 requires python 3.9 or later. Should we drop support for 3.8? @khintz @leifdenby

I would be fine with dropping 3.8

[SimonKamuk]

Oh it looks like we already require python>=3.9, we probably just forgot to remove the test for 3.8. I've done that now.

[leifdenby]

Looks great! Thanks for doing this ⭐