Document support for the free-threaded build and add to tests

[ngoldbaum]

How would this feature be useful?

AFAIK, nox works fine on the free-threaded build, and I've been using it in the CI for several projects.

Describe the solution you'd like

I notice that you aren't running tests using 3.13t, it's probably worth adding that and seeing if anything is broken.

If that works, then it would be nice to officially document that you support the free-threaded build and that it should be referred to as e.g. 3.13t with a t suffix.

Describe alternatives you've considered

No response

Anything else?

No response