Replace TravisCI with GitHub Actions.

[ruebot]

GitHub issue(s): #506

What does this Pull Request do?

Replace TravisCI with GitHub Actions.

How should this be tested?

You can see it in action here: https://github.com/ruebot/aut/runs/1398762512?check_suite_focus=true

...and it should all trigger below. Don't worry about TravisCI not running.

[codecov[bot]]

Codecov Report

Merging #507 (d0d2683) into main (8104a65) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #507   +/-   ##
=========================================
  Coverage     88.83%   88.83%           
  Complexity       57       57           
=========================================
  Files            43       43           
  Lines          1012     1012           
  Branches         85       85           
=========================================
  Hits            899      899           
  Misses           74       74           
  Partials         39       39