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 3.4 because GitHub Actions doesn't fully support it. It's EOL anyway.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6ec1350). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #137   +/-   ##
=========================================
  Coverage          ?   72.58%           
=========================================
  Files             ?        9           
  Lines             ?      434           
  Branches          ?       54           
=========================================
  Hits              ?      315           
  Misses            ?      114           
  Partials          ?        5           

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