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: python-pillow/Pillow#5028.

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

Once difference: GHA doesn't have allow_failures like Travis does, and the 3.10-dev (which is the closest to nightly on Travis) is failing because Wheel doesn't yet support 3.10: pypa/wheel#354. So that's commented out for now.

Example build:

• https://github.com/hugovk/pillow-perf/actions/runs/365836504

[nulano]

Once difference: GHA doesn't have allow_failures like Travis does, and the 3.10-dev (which is the closest to nightly on Travis) is failing because Wheel doesn't yet support 3.10: pypa/wheel#354. So that's commented out for now.

There is https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#jobsjob_idcontinue-on-error, but it's not entirely clear to me what it does from the documentation and I haven't tried it myself.

[hugovk]

Currently, using ‘continue-on-error:’ in a step flags the step with a green check and allows the job to continue. The only way to see if the step failed is to look at the step’s log.

https://github.bokerqi.topmunity/t/continue-on-error-allow-failure-ui-indication/16773

Also: actions/runner#2347

[homm]

Do we need to enable Actions somehow for the repo first to see this config in action? )

[hugovk]

It should be enabled on merge.

Or you could try pushing my fork's branch to a branch in this repo, that might do it.

You can see a test run https://github.com/hugovk/pillow-perf/actions/runs/365836504

If you prefer, we could keep .travis.yml in this PR, merge, and remove it in a followup PR.