Fix issue #418. Unpin pymbolic. Resolve pydantic deprecation warnings

[huard]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #418
• [ ] (If applicable) Documentation has been added / updated (for bug fixes / features).
• [ ] (If applicable) Tests have been added.
• [x] CHANGELOG.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?

• Unpin pymbolic
• Resolve issue with pymbolic > 2022.2
• Tweaks to silence pydantic DeprecationWarning

Does this PR introduce a breaking change?

Python3.9 support has been dropped.

Other information:

[Zeitsperre]

@huard If you want to implement a drop of Python3.9 in order to address the problem, go for it. We've been progressively dropping that version in a few places already.

[coveralls]

coverage: 81.279% (-0.07%) from 81.352% when pulling ede6f94acf340856b005a4a6347c92da0ed378f7 on fix-418 into 57b7df4cf6009b5d62b8ce3a8ba62593dd55e619 on master.