Migrate to GitHub Actions

[jezdez]

Travis CI has a new pricing model which places limits on open source.

• https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
• https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

Many projects are moving to GitHub Actions instead, including Jazzband projects:

• https://github.com/orgs/jazzband/projects/1
• Should we disable travis? help#206

This is based on jazzband/contextlib2#26.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

Merging #72 (28b6e4c) into master (6c82e3d) will increase coverage by 2.75%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   39.00%   41.76%   +2.75%     
==========================================
  Files           7        7              
  Lines         423      443      +20     
  Branches       78       78              
==========================================
+ Hits          165      185      +20     
  Misses        250      250              
  Partials        8        8              

Impacted Files	Coverage Δ
authority/decorators.py	44.06% <66.66%> (+2.99%)	⬆️
authority/templatetags/permissions.py	29.80% <0.00%> (+6.24%)	⬆️

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[jezdez]

Thank you @auvipy!