Replace Travis CI with GitHub Actions

[hugovk]

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
• https://github.com/jazzband-roadies/help/issues/206

This is based on https://github.com/jazzband/contextlib2/pull/26.

It drops support for Python 3.3 and 3.4 because GitHub Actions doesn't fully support them. They're EOL anyway.

Python 3.9 support is also added.

No more mention of Travis CI in the codebase:

$ git grep -i travis
$

TODO:

• [x] @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.
• [x] @jezdez to remove continuous-integration/travis-ci and coverage/coveralls from required checks
• [x] @jezdez Adding GHA and Codecov as required checks after merge

[jezdez]

@hugovk Sorry for the mess with the Jazzband implementation PR #134!

[hugovk]

No problem, rebased!

[jezdez]

I'm wondering why no action is run for this PR though?

[hugovk]

I think it needs at least some workflow file in the repo before it's activated for the very first time.

[hugovk]

~And this is the passing build from my fork: https://github.com/hugovk/jsonmodels/actions/runs/383954700~

[hugovk]

This is the passing build from my fork: https://github.com/hugovk/jsonmodels/actions/runs/383955129

[jezdez]

Gotcha, let's merge this. Thanks again @hugovk!

[jezdez]

I've re-enabled the checks requirement (test builds and codecov) in the master branch protection settings.