Add GitHub Actions CI

[alexrudd2]

pytest was missing as a test dependency, so this adds that. Oops, it's unittest

It also sets up a working GitHub Actions CI, which is free for public repos. I understand that Evil Mad Scientist has an internal CI server somewhere, but this is public and helps other contributors.

Note that 3.9.2 hasn't been released
on GitHub yet so the CI manually downloads 3.9.0.

[alexrudd2]

You can see it run on my fork.