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.

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@41a0379). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #32   +/-   ##
=========================================
  Coverage          ?   43.60%           
=========================================
  Files             ?        7           
  Lines             ?      133           
  Branches          ?       20           
=========================================
  Hits              ?       58           
  Misses            ?       72           
  Partials          ?        3           

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[jezdez]

@martey Hey there, just a heads-up that I've also asked a review from a few other Jazzband members that have been helping with the big move to GitHub Actions in other Jazzband projects.

FWIW This won't change much for this project, except actually enables the Jazzband release pipeline for you. Please let me know if you have any questions.

[martey]

Thanks @jezdez!