Move to GitHub actions

[robertodr]

• CI for Linux and macOS are now run through GitHub actions rather than Travis (for Linux) and Azure (for macOS)
• Code coverage analysis is done in a separate action.
• I didn't manage to move Windows to GitHub actions. We might want to retry adding windows-latest to the OS matrix at a later stage.

[codecov-io]

Codecov Report

Merging #145 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   53.99%   53.99%           
=======================================
  Files          91       91           
  Lines        2415     2415           
=======================================
  Hits         1304     1304           
  Misses       1111     1111           

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[robertodr]

Feel free to squash the commits.