Test multiple Python versions in github workflow

[notmatthancock]

If we are not explicitly requiring a specific Python version (and I don't think we should since this software is open source, where pinning to a specific minor version would feel too restrictive), then we should test multiple versions in the github workflow test runner.

This would be a simple update by adding a matrix strategy block (for example):

...
    strategy:
      matrix:
        python-version: ["pypy3.9", "pypy3.10", "3.9", "3.10", "3.11", "3.12"]
...