Add Github actions testing

[meson800]

This converts the existing travis CI to Github CI, possibly closing #108. I plan on possibly submitting more PRs, so set this up on my fork to test my edits and thought it could be useful here! Github actions are free for public repos too :)

Notes

• The flake run shows a lot of warnings, so not sure if this correctly pulls in any necessary flake config.
• Coveralls is not setup because I don't have a coveralls instance but it should be fairly easy to setup; I have used codecov before through GHA and all you really need is a three-line step at the end. It looks like Coveralls needs GITHUB_TOKEN set so it can comment on PRs.

    - name: Coveralls
      if: always()
      uses: coverallsapp/github-action@master
      with:
        github-token: ${{ secrets.GITHUB_TOKEN }}

The if: always() line means that we can "fail the GHA build" with a failing test, but it will still always upload coverage to Coveralls.

[canismarko]

Thanks for the PR.

The flake run shows a lot of warnings, so not sure if this correctly pulls in any necessary flake config.

No idea. I keep meaning to setup a pre-commit hook to format using black, so that might fix some of the warnings.