Test `rustworkx` with free-threaded Python

[IvanIsCoding]

What is the expected enhancement?

Starting with 3.13, we should test rustworkx with https://py-free-threading.github.io/ to make sure it at least work with a single-thread setting. I don't think we should make our data structures thread-safe (that is a lot of overhead).

Of course this will heavily depend on PyO3 and their API offerings. But so far they have made progress on removing the GIL

[IvanIsCoding]

And we are blocked until NumPy releases a new version supporting free threaded Python anyway: https://py-free-threading.github.io/tracking/.

[gulldan]

https://github.com/numpy/numpy/releases/tag/v2.1.0