Support free-threading Python wheel distributions

[andfoy]

As some of you might be aware, since CPython 3.13, there is experimental support for an alternative interpreter distribution (via PEP703) that disables the GIL altogether, thus enabling support for real thread parallelism.

Since PyTorch now supports free-threaded wheel distributions (pytorch/pytorch#130249), it should be easy producing wheels for fairscale as well.

In order to provide free-threaded wheels, there is a need for extensive testing, which can be summarized in performing the following tasks:

• Audit Python bindings and declare them free-threading compatible (xref https://py-free-threading.github.io/porting/#updating-extension-modules).
• Run the test suite with pytest-run-parallel to find potential issues, and fix them.
• Run the test suite under ThreadSanitizer. Should be feasible, since there are no runtime dependencies aside from Cython.
• Add cp313t-* to CI to build free-threading wheels.

I can volunteer in taking charge of these tasks if necessary