Move CI to 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, and we're moving the python-pillow repos over: https://github.com/python-pillow/Pillow/issues/5028.

This PR removes the Travis CI testing and does the same on GitHub Actions.

Once difference: instead of 16.04 Xenial, it uses ubuntu-latest which is currently 18.04 Bionic and soon 20.04 Focal.

Example build:

• https://github.com/hugovk/Sane/actions/runs/365649997

[hugovk]

There's been no objections so I'll merge this, as the situation at Travis is getting worse.

[manisandro]

Sorry missed this one - thanks for taking care of this!

[hugovk]

You're welcome!