Use tox to run pytest-copie unit tests locally on multiple python versions

[drewoldag]

Change Description

This proof of concept PR shows that we can use tox with a simple configuration file to execute the pytest-copie-based unit tests locally on a group a set of different python versions.

There is some amount of dev environment setup required:

1. Install tox
2. Use brew install python@3.xx for any version of python that is not installed on your system 3.8 - 3.12.

Note that when running tox with the tox.ini file as defined, if you are missing a version of python that tox has been instructed to use, it will simply skip that version and log a warning.

Checklist

• This PR is meant for the lincc-frameworks/python-project-template repo and not a downstream one instead.
• This change is linked to an open issue
• This change includes integration testing, or is small enough to be covered by existing tests

[delucchi-cmu]

I'm not sure how often I would personally use tox, because I'm a little too lazy. But this looks like a great addition!

[drewoldag]

@delucchi-cmu yours is a very fair point. There is certainly more effort involved in setting up tox to run. However, once you have tox installed in your virtual environment (even if you haven't added multiple additional python versions), running tox at the command line will be very similar to running pytest tests at the command line.

tox will simply skip over the versions of python that it can't find, but will run the tests for the versions that it can find.

Putting all that aside though, I really want to have more confidence when we assert "The PPT works for Python >= 3.8".