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
• https://github.com/jazzband-roadies/help/issues/206

This is based on https://github.com/jazzband/contextlib2/pull/26.

Fixes #187.

TODO:

• [x] @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[claudep]

Great! Did you leave travis config file on purpose?

[jezdez]

@claudep Yeah, not done yet :)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@908359d). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #188   +/-   ##
=========================================
  Coverage          ?   93.64%           
=========================================
  Files             ?       10           
  Lines             ?      519           
  Branches          ?       65           
=========================================
  Hits              ?      486           
  Misses            ?       20           
  Partials          ?       13           

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