Use Github Actions + cibuildwheel for all wheels

[hauntsaninja]

Besides reducing the number of places wheels end up, cibuildwheel does nice things like run auditwheel for you. We also run check-manifest so our sdist is more reliably packaged. Updates to documentation.

[hugovk]

Very good!

AppVeyor was running the tests, should GHA run tests too?

And can .travis.yml be removed?

[hugovk]

I recommend removing this branch restriction, so contributors can test their feature branches before opening PRs.

Relatedly, do you have an example build?

[hauntsaninja]

Thanks for the feedback!

Example build here: https://github.com/hauntsaninja/typed_ast/actions/runs/1327600384
I changed things to also run tests, so now we can get rid of travis.
I put in the branch restriction because I had the workflow create a github release containing the wheels. But that's more useful for the project I was basing this off of, for typed_ast we can just go to the relevant github actions page and download the artifacts.

[hugovk]

Looks good!

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It's also possible to upload the sdist+wheels to PyPI as part of a release (e.g. tag) using something like https://github.com/pypa/gh-action-pypi-publish, but I'd suggest leaving that for a followup and making a release first with this.