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 < 3.6 because GitHub Actions doesn't fully support them. It's EOL anyway.

Python 3.9 support is also added.

Also Django support was moved ahead quite a bit to only support >= 2.2.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

Merging #72 (c1b77d9) into master (5b253e6) will decrease coverage by 6.30%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   95.83%   89.52%   -6.31%     
==========================================
  Files           5        4       -1     
  Lines         312      296      -16     
  Branches        0       60      +60     
==========================================
- Hits          299      265      -34     
- Misses         13       15       +2     
- Partials        0       16      +16     

Impacted Files	Coverage Δ
menu/__init__.py	71.42% <71.42%> (-28.58%)	⬇️
menu/menu.py	90.90% <100.00%> (-6.00%)	⬇️
menu/tests/test_menu.py	100.00% <100.00%> (ø)
menu/templatetags/menu.py	61.70% <0.00%> (-19.15%)	⬇️

---

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[jezdez]

Obviously this is quite the step, but the old Python and Django versions aren't really supported anymore and we should use the oppurtunity to make the jump.