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:

• [request] support --disable-gil builds (PEP703) cibuildwheel#1657
• Support --disable-gil builds (PEP 703) in packaging.tags packaging#728
• --disable-gil builds install incompatible stable ABI extensions python/cpython#111870

[mayeut]

This will also require pypa/pip#12300 (and a new packaging release before that).

Edit: this is mentioned but not referenced in python/cpython#111870 (comment)