Support for an experimental version of free-threaded (no-gil) Python 3.13

[joerick]

PEP 703 was accepted recently, meaning that free-threaded Python is moving into a new phase, where the goal is to see how it would really work in the Python ecosystem, with real C extensions! A large part of the unknowns of the project lie in how the C extension world will work, when there's no global lock protecting Python objects.

Enabling wheel-building seems like it might help accelerate this kind of testing. I think normally support for interpreters is only added at the beta phase, but perhaps the situation would warrant a special case?

refs:

• https://github.com/pypa/cibuildwheel/issues/1657
• https://github.com/pypa/packaging/pull/728
• https://github.com/python/cpython/issues/111870