Add Python 3.9 Support

[wouterjdb]

This PR tests the usage of Python 3.9

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Contributor checklist

• [ ] :tada: This PR closes #ISSUE_NUMBER.
• [ ] :scroll: I have broken down my PR into the following tasks:
  • [ ] Task 1
  • [ ] Task 2
• [ ] :robot: I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• [ ] :book: I have considered adding a new entry in CHANGELOG.md.
• [ ] :books: I have considered updating the documentation.

[wouterjdb]

ERROR: Could not find a version that satisfies the requirement ecl>=2.9 (from flownet)
ERROR: No matching distribution found for ecl>=2.9
Error: Process completed with exit code 1.

[wouterjdb]

ecl 2.10.1 now exists with py39 support

[wouterjdb]

equinor-libres doesn't have Python 3.9 support yet.

[wouterjdb]

https://pypi.org/project/equinor-libres/9.0.1/ now supports Python 3.9.

[wouterjdb]

pyvista, or actually its dependecy vtk is not yet supported on Python 3.9.

[wouterjdb]

Pyvista should now support Python3.9: https://github.com/pyvista/pyvista/releases/tag/0.31.3

[wouterjdb]

The new pip dependency resolver results in very slow installation performance. A quick solution for this could be to use the legacy dependency resolver, like so: --use-deprecated=legacy-resolver

[wouterjdb]

XTgeo doesn't support 3.9 yet - but the pre-releases do.... Shouldn't take too long: https://github.com/equinor/xtgeo/releases