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 #174 (516306c) into master (9a322c4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files          12       12           
  Lines         377      377           
=======================================
  Hits          347      347           
  Misses         30       30           

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[aleksihakli]

A few notes on the test matrix configuration and badges, looks good aside from those.

[aleksihakli]

Reviewed and LGTM 👍

[kencochrane]

Thank you @jezdez @aleksihakli