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.

It drops support for Python 3.5 because it's EOL anyway.

Python 3.9 support is also added.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

Merging #126 (84f946b) into master (99dd6f0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   89.28%   89.28%           
=======================================
  Files          10       10           
  Lines         280      280           
  Branches       31       31           
=======================================
  Hits          250      250           
  Misses         22       22           
  Partials        8        8           

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