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.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

Merging #462 (382110f) into master (775ac3f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #462      +/-   ##
==========================================
+ Coverage   94.58%   94.59%   +0.01%     
==========================================
  Files           6        6              
  Lines         757      759       +2     
==========================================
+ Hits          716      718       +2     
  Misses         41       41              

Impacted Files	Coverage Δ
model_utils/__init__.py	100.00% <100.00%> (ø)

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[charettes]

Except for the 2.1 references this LGTM

[jezdez]

Thanks for the quick review, all! ❤️

[charettes]

Worked flawlessly https://github.com/jazzband/django-model-utils/runs/1470115542