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 GitHub Actions doesn't fully support them. 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@f2490ba). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #607   +/-   ##
=========================================
  Coverage          ?   68.30%           
=========================================
  Files             ?       24           
  Lines             ?     1101           
  Branches          ?      173           
=========================================
  Hits              ?      752           
  Misses            ?      312           
  Partials          ?       37           

---

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 f2490ba...3612c9c. Read the comment docs.

[jezdez]

Thanks @auvipy!