Remove Python 3.8 support and fix other requirements

[RondeauG]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #150
  • This PR fixes #175
• [x] (If applicable) Documentation has been added / updated (for bug fixes / features)
• [x] This PR does not seem to break the templates.
• [x] HISTORY.rst has been updated (with summary of main changes)
  • [x] Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Drops support for Python 3.8
• Unpins xarray's version

Does this PR introduce a breaking change?

xscen no longer support Python 3.8

Other information:

[Zeitsperre]

Don't forget to adjust the black and pyupgrade configurations in .pre-commit-config.yaml!