Add initial GitHub Actions testing

[tianon]

This mirrors .travis.yml fairly closely, but gives us a much newer 3.10-dev version (specifically 3.10.0-beta.1, at this time of writing, which is relevant for #2061).

Per #2061, I've set 3.10-dev to "allow failure" (since it's known-broken ATM), and I've currently got PyPy set similarly since it fails for reasons I wasn't able to chase down completely (that I think are due to how https://github.com/actions/setup-python is installing/configuring PyPy, but I'm not 100% certain).

You can see a "successful" run of this at https://github.com/tianon/hy/actions/runs/829829568. 😄

As you can see there, it takes ~2m to run this entire matrix, and it triggered almost instantly every time I force pushed.

Relevant links:

• https://docs.github.com/en/actions/guides/building-and-testing-python
• https://github.com/actions/setup-python
• https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions

[Kodiologist]

I like the sound of this. Would it be too much to ask you to fix the 3.10 bug and get PyPy working again as part of this PR? If so, I guess I'll try instead.

[Kodiologist]

I guess so. I'll take a shot.

[tianon]

Sorry, I've been trying to get back to this and haven't had a minute (and frankly fixing the 3.10 issue feels a bit over my head). 😬

[allison-casey]

looks like we weren't constructing an alias via asty.alias so lineno and col_offset weren't set like 3.10 expects now. also pypy works for me so maybe the changes fixed both? Anyways how do we want to do this? Merge this then merge merge my fix? I don't have github actions running on my fork atm so that might be easiest

[Kodiologist]

I'm still working on this. It's mostly done. I'll make a new PR when it's ready.

[Kodiologist]

Superseded.