Use pattern matching for config schema

[eivindjahren]

Use pattern matching now that we no longer support python 3.9

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[berland]

This change will make it slightly more awkward to backport any future critical bugfix into the 3.8 code.

[eivindjahren]

This change will make it slightly more awkward to backport any future critical bugfix into the 3.8 code.

Yes, we talked a bit about this at the office. We can discuss it after lunch and decide what to do then.

[eivindjahren]

We will just wait until backporting python 3.8 fixes is less urgent.