chore: build free-threading wheels

[Rogdham]

We don't support free-threading, but this should not prevent us from shipping wheels.

>>> import sys
>>> sys._is_gil_enabled()
False
>>> import pyzstd
<frozen importlib._bootstrap>:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'pyzstd._c._zstd', which has not declared that it can run safely without the GIL. To override this behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.
>>> sys._is_gil_enabled()
True