Add Python 3.9 to Github workflow

[olelod]

Insert a description of your pull request (PR) here, and check off the boxes below when they are done.

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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]

This is too early due to opm not yet having Python 3.9 support. To be continued....