Add CI tests for latest dev Python build

[micahellison]

Feature Request

Back when we used Travis, we had an extra build step for running all tests in the latest development version of Python. It looks like we lost that step during the migration to GitHub Actions. It would be good to have it back.

Use Case/Motivation

This tightens our feedback loop regarding potential issues in future versions of Python, long before they are released.

Other information

The resolution to #1270 "solves" this in the short run, but ideally we'd just have a version-agnostic step for the latest dev version so we wouldn't have to be nudged every time there's another major Python release coming.

[micahellison]

Closing this since the solution to #1270 works, and it's fine for us to have to manually to change a little CI configuration once a year or two to support the next Python version.