action: use a venv to prevent PEP 668 errors

[woodruffw]

WIP.

Closes #144.

[woodruffw]

This is good to go. To summarize:

We now isolate this action's Python dependencies inside of a virtual environment, rather than using either the distribution or configured Python's user packages site. This makes us both compatible with PEP 668 (the source of the error) and avoids the (slim) likelihood of conflict with other steps running in the same job.

(Plumbing everything through this new venv is a minor adventure, since Windows and *nix venvs have slightly different directory layouts. But the selftests confirm that the specialization for both works.)

[jku]

Wow, This looks surprisingly complicated to deal with. I don't have a better suggestion so let's go with that