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

Python 3.6, 3.7, 3.8 and 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@f07707c). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #16   +/-   ##
=========================================
  Coverage          ?   30.35%           
=========================================
  Files             ?        8           
  Lines             ?      313           
  Branches          ?       51           
=========================================
  Hits              ?       95           
  Misses            ?      216           
  Partials          ?        2           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f07707c...84b12f6. Read the comment docs.