Fix importlib.resources deprecations for py 3.9

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

Fix for the deprecation warnings that importlib.resources throws. It changes how we open and parse our module data files.

Does this PR introduce a breaking change?

Yes, the deprecation concerns python 3.9 and above, so this needs to be merged only when we pin python above 3.8.

Other information:

I made this PR so I don't lose the changes I had already made, but there's no rush at all.

[github-actions]

Note
It appears that this Pull Request modifies the main.yml workflow.

On inspection, the XCLIM_TESTDATA_BRANCH environment variable is set to the most recent tag (v2023.9.12).

No further action is required.

[Zeitsperre]

@aulemahal Want me to finalize this here? We can get this into v0.46.0 for sure

[aulemahal]

Ah! Now that you added retrocompatibility, why not!