Support free-threaded Python [3.14]

It's no longer marked experimental, as of 3.14 now in rc2.

https://py-free-threading.github.io/running-gil-disabled/
>The free-threaded Python executable will always have a python3.13t alias (for Python 3.13); whether python, python3 and/or python3.13 point at the free-threaded executable depends on the installation method used (see the install guide for more details).

I believe it's off by default in 3.13, but should also be supportable there.

It's a question if JuliaCall should install and use those t versions. I'm not sure if it means any changes for JuliaCall, other than pointing to the right runtime.