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.

Python 3.9 support is also added.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3b7a954). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             master      #123   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?        10           
  Lines             ?       395           
  Branches          ?        59           
==========================================
  Hits              ?       395           
  Misses            ?         0           
  Partials          ?         0           

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[timgraham]

Please add trove classifier for Python 3.9 and possibly a release note ala 6590a87.