correctly match free-threaded python versions

[zzzeek]

in the case where nox is run given an explicit python version target to run on, nox locates Python interpreters by matching that interpreter string to a regex, that is then expanded to match the name of a Python executable by appending the numeric portions of the version to the word "python", its existence is verified with
shutil.which(), and is then passed off to virtualenv as a target to use.

However, if the explicit python version ends in "t" for a free-threaded python, such as "3.13t", the regex does not match, due to that "t" that's present which isn't included in the regex.

This PR fixes the regex to also look for a "t" at the end for a free-threaded version name, and formats it into the interpreter name virtualenv will look for.

There's a bit of a secondary issue which is that if you are trying to select for "3.13" non-free-threaded, but you have a free-threaded 3.13t in your path as well, that "python3.13" name matches in the free-threaded bin folder as well, but virtualenv seems to know how to make the right choice in this case.

The only issue I could find regarding free-threading is #949 which vaguely claims that free-threading is working, however this patch is needed if you actually want to indicate the Python version.

Fixes: #949

[zzzeek]

TIL I can also send the full executable name like python3.13t in, and that bypasses the regex issue here, but this still would be a good fix to have

[zzzeek]

ah it fails when we use pytest-xdist and run in separate processes.

[henryiii]

Part of #949, I think.

[ngoldbaum]

Thank you for fixing this!

I had only used nox with projects that don't use any of the python environment setup features and just pip install nox into a single Python environment. So I never ran into the problems solved here.