Add GitHub Actions build.yml template

[foolip]

Also remove any files that are part of the Travis setup.

Part of whatwg/meta#173.

[domenic]

This probably needs exceptions for streams and encoding at least, to not generate a deploy.yml for them since they are special.

[foolip]

It looks like encodings is different only in that it requires Python 3.8, but https://github.com/actions/virtual-environments/blob/master/images/linux/Ubuntu2004-README.md already has 3.8. It's not as locked down as picking a version we want using actions/setup-python@v2, but is likely to work fine in practice. The same with anything needing node, actually, since some version is available by default.

[foolip]

For streams there's already an exception to not process it at all in factory.py, so that one's OK.

[andreubotella]

For streams there's already an exception to not process it at all in factory.py, so that one's OK.

That shouldn't stay there for much longer, since whatwg/streams#1035 is now merged.

[foolip]

I've tweaked this a bit and it seems to work when run:
whatwg/encoding#218
whatwg/fullscreen#175

Sort of, one can tell from the directory names that something's wrong.

[foolip]

I've fixed a problem and created some more PRs using the branch which look good:
whatwg/compat#126
whatwg/console#178
whatwg/dom#872
whatwg/fetch#1037

I think this is ready to land now, I don't want to create more PRs without landing the existing ones and confirm them working, in case some additional tweaks are needed.

[annevk]

This looks okay to me, but when looking at whatwg/dom it didn't seem to be working to me.

[foolip]

Blush. After the first two worked I eagerly commented before the second two had finished on the assumption they would run. I'll investigate why the did not.

[foolip]

Explaining the appearance of Travis being stuck on multiple PRs, as in whatwg/compat#126 (comment).

For most repos the Travis status is required by branch protection. That makes the status appear to be pending even though it won't be triggered. For each repo I handle, I'll update the branch protection as I land the change.