Added github actions workflow to replace travis

[lostmsu]

Subj

This drops coverage for setup.py

[lostmsu]

Either we:

1. make our own docker images to test on (not sure how this works)
2. have the build take 6+ minutes instead of 1 (remove Mono 6.12 and reinstall 5.20 instead)
3. or switch Mono to 6.12 for testing

Considering we are doing a major version bump, we can pick option 3 for simplicity. @filmor thoughts?

[filmor]

I'm in favour of just using the latest version.

[codecov-io]

Codecov Report

Merging #1307 (d47d388) into master (5b09ba6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1307   +/-   ##
=======================================
  Coverage   74.04%   74.04%           
=======================================
  Files           1        1           
  Lines         289      289           
=======================================
  Hits          214      214           
  Misses         75       75           

Flag	Coverage Δ
setup_windows	74.04% <ø> (ø)

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