Move away from Travis CI

[gsnedders]

Probably to GitHub Actions?

While we're rewriting the config, we may well want to migrate to cibuildwheel at the same time, especially before #445 happens (when we'll then need the infra to build non-universal wheels for releases cross-platform).

[jayaddison]

GitHub actions is probably the most straightforward alternative at the moment, yep.

Could we build universal wheels? I think that'd allow building and publishing wheels without introducing additional build-time dependencies.

Thoughts on build configuration:

During migration, running tox is likely the simplest way to run tests under all of the configured environment combinations. That would run all of the tests sequentially in each of the environment configurations and output a single pass/fail indicator in GitHub for the developer.

Perhaps it'd make sense during the migration to have the chosen continuous integration system run each of the configured environments as a separate build (i.e. individual process with pass/fail indicator)?

There are a reasonably large number of environment combinations already (21 by my count?), so we'd want to be careful not to create redundant duplicate work - especially if opening pull requests causes all those workloads to be initiated (resulting in expenditure of compute resources (and therefore energy :)) that it'd be better to conserve).

[jayaddison]

Hrm, mapping GHA strategy.matrix-based jobs to single tox environments looks a little tricky due to the combinatorial envlist.

[hugovk]

GitHub Actions was added in #527, and tests are also run on AppVeyor for Windows (this could also be moved to GHA, but that's another issue).

Also Travis CI hasn't been running for 10 months:

https://travis-ci.org/github/html5lib/html5lib-python/builds

Please see #545 to remove Travis CI config and replace with GHA in docs.