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 2.7 and 3.5 because GitHub Actions doesn't fully support them. They're EOL anyway.

Python 3.9 support is also added.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

This also makes use of setuptools_scm going forward, which means you'll only need to create a git tag to update the version information for packaging and documentation. Changelog etc still need to be updated regularly.

This also switches from Coveralls to Codecov to stay consistent with other Jazzband projects.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1fbb43b). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #323   +/-   ##
=========================================
  Coverage          ?   80.42%           
=========================================
  Files             ?        8           
  Lines             ?      470           
  Branches          ?       68           
=========================================
  Hits              ?      378           
  Misses            ?       60           
  Partials          ?       32           

---

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