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 it's EOL.

Python 3.9 support is also added.

TODO:

• @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov]

Codecov Report

Merging #703 (d15be01) into master (1e4dca3) will decrease coverage by 3.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #703      +/-   ##
==========================================
- Coverage   94.67%   91.43%   -3.25%     
==========================================
  Files           3        3              
  Lines         432      432              
  Branches       78       78              
==========================================
- Hits          409      395      -14     
- Misses         14       23       +9     
- Partials        9       14       +5     

Impacted Files	Coverage Δ
taggit/managers.py	90.77% <0.00%> (-4.04%)	⬇️

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[jdufresne]

Thanks!

The dropping of Python 3.5 support should be added to the CHANGELOG.rst

[hugovk]

🎈

[aleksihakli]

A few suggestions on the tox.ini configuration.

[jezdez]

Thanks all for the reviews! 😍