Add support for the free-threaded build

blosc2 ships free-threaded wheels, but they re-enable the GIL on import:

```
goldbaum at Nathans-MBP in ~
○ python
Python 3.14.2 free-threading build (main, Dec 8 2025, 15:41:12) [Clang 17.0.0 (clang-1700.4.4.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
i>>> import blosc2
:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'blosc2.blosc2_ext', 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.
```

See https://py-free-threading.github.io for more information about the free-threaded build and how to support it.

Empieza reproduciendo la advertencia de importación con la compilación free-threading de Python 3.14.2 e inspecciona cómo se compila y carga blosc2.blosc2_ext. Lee las indicaciones en py-free-threading.github.io; el trabajo estará terminado cuando se pueda importar blosc2 sin volver a habilitar el GIL en una compilación free-threaded.

Escrito por el modelo de indexación a partir del texto del issue.