Unit tests are only run against a single version of Python on the `main` branch.

[fpgmaas]

I think it could be useful to run unit tests against multiple versions of Python on each PR, to gain extra confidence in the proposed changes of a PR. This also builds extra confidence that chispa actually works on the python versions as specified in pyproject.toml, and could prevent issues such as #78 in the future.

I have created a draft PR to solve this; #81