Switch to GitHub Action & GitHub Packages, test on more Python versions

[alanbriolat]

• Replace Travis CI with GitHub Actions
• Replace Docker Hub builds with GitHub Actions + push to GitHub Packages
• Add Python 3.8 and 3.9 to the test matrix
• Switch from coveralls.io to codecov.io, because the latter plays nicer with GitHub Actions by default

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@38ebd6f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #171   +/-   ##
=========================================
  Coverage          ?   73.48%           
=========================================
  Files             ?       29           
  Lines             ?     3043           
  Branches          ?      428           
=========================================
  Hits              ?     2236           
  Misses            ?      741           
  Partials          ?       66           

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[coveralls]

Coverage remained the same at 74.856% when pulling 52791c3 on github-actions into 38ebd6f on master.