Fix union use in utils.py & add tox GitHub workflow

[jordan-gillard]

This PR fixes a Python 3.9 incompatible type annotation in utils.py. It also adds tox to build the project and run a basic test using Python versions 3.9, 3.10, 3.11, and 3.12. I added a GitHub workflow as well to run these checks in CI. This will keep Python 3.9 incompatible changes from being reintroduced in the future.

Resolves: #32
Relates to: #11

[jordan-gillard]

@GalenReich @andy-seleya

[GalenReich]

@jordan-gillard you are my hero

I have some best practice questions about when is a good time to increment version numbers (I am notionally trying to stick to semver but feel I probably increment the patch number too frequently)... any advice?

[GalenReich]

Ooh, maybe we should add a line to the README, something like:

# Run the tests
poetry run tox

[jordan-gillard]

Ooh, maybe we should add a line to the README, something like:

# Run the tests
poetry run tox

Ooo yeah, good call. Updated in c7752c1

[jordan-gillard]

I have some best practice questions about when is a good time to increment version numbers (I am notionally trying to stick to semver but feel I probably increment the patch number too frequently)... any advice?

Haha honestly we're both on the same page. I just try to use the Semver 2.0.0 Standard as best I can. Also, it's totally fine to frequently increment the patch version. That's how it should work :)

I have used tools in the past that automatically bump project versions but I never felt it was worth it. I'd say for now it's okay to just do it manually as best we can. For a small project like this one with only a few maintainers, manual updates are probably best.