Add basic CI checks via GitHub Actions to run tests and validate packaging

[CAM-Gerlach]

As originally mentioned in PR #21

Here, we can use a simplified version of our basic workflow template, as used on many of our other repos. QtPy (e.g. as of spyder-ide/qtpy#272 ) is the most recent example, though in that case (like most of the other extant ones, for various specific reasons), we used Miniconda to set up the envs and a bash script to run through them, so we could test different Qt binding versions on the same job. Here, that could be greatly simplified to something most closely resembling what I have in Sub Manager (which was itself based on the previous Spyder repo GitHub actions I developed), though with the further improvements made to those in QtPy and elsewhere.

More or less blocked on #23