Add multiple python versions to CI tests

[shaunagm]

We had a pair programming session where we attempted to address #790 within CircleCI, that was unsuccessful so we switched to Github Actions.

• [x] get tests working in github actions
• [x] add caching
• [x] move Windows error to new issue #860
  • see this windows error
• [x] check usage requirements to make sure we won't run out of minutes/storage
  • a single run of the tests (both Linux and Mac) is about 5 minutes, and we seem to have 2000 free minutes total, so we have about 400 runs per month or an average of 12-13 test runs per day average. that may not be enough but it may be fine? Buying additional minute seems to cost about $10 per thousand minutes so it's not a huge deal if we overrun occasionally
• [x] check that the docs build stuff is still working on circle CI

[shaunagm]

Thanks for the feedback @gcollazo! I updated to address suggestions 1 & 3. Regarding question 2 - we have a set number of minutes we can use per month, and tests on Mac are given a 10x multiplier, so something that takes 3 minutes costs 30 minutes (as opposed to 3 minutes on Linux, and 6 minutes on Mac). I don't think we have enough free minutes to actually run that many Mac tests, so I'm hoping that just running on one version of Python will catch any Mac-specific errors.