Add github actions CI

[moble]

This PR attempts to replicate the CircleCI workflow (which is not working as of this writing) in github actions. It also removes references to CircleCI.

It also fixes a few minor things I needed to do to get the tests to pass:

• Replace the now-deprecated getargspec with getfullargspec in galgebra/lt.py
• Set self.nargs = 0 in the branch following the getargspec usage, so that it's not referring to a non-existent variable
• Fix an if statement in galgebra/mv.py so the linter doesn't choke
• Fix the python_requires syntax in setup.py

This is one half of the split version of #486. The other half is a branch on my fork https://github.com/moble/galgebra/tree/jupyterlite, which starts relative to this PR's first commit; I'll make it into a PR once this is merged.

[moble]

Okay, I believe I've addressed the comments above.

The tests passed here.

[utensil]

Why is the doc build failed?

Other than this and the need to resolve the conflict, all LGTM.

[moble]

Why is the doc build failed?

Looks like we were one of the lucky few to hit a brownout requiring the new .readthedocs.yaml format. I'll try to update it.

[utensil]

As everything looks good to me, and I wish to polish CI and bump Python etc. Merging this as the foundation. Thanks!