Enhance no GIL / free threaded support

CPython now has free-threaded builds since 3.13: https://docs.python.org/3/howto/free-threading-python.html

In https://github.com/openzim/python-libzim/pull/237 (to be released as 3.8.0), we've fixed tests and documentation regarding the fact that python-libzim requires the GIL ATM.

We need to investigate if we really need the GIL, because multiple methods are already marked as nogil, especially important ones around the creation of the ZIM (where we do expect the python-libzim user to handle thread-safety when required).

Aim would be to mark the module as free-threaded. I don't know yet how to do this with Cython (if possible at all).

Inizia dalla documentazione sul free-threading di Python e dai metodi esistenti contrassegnati con nogil. Verifica se Cython può contrassegnare il modulo come free-threaded e se il binding richiede effettivamente il GIL. Il lavoro è completato quando il modulo supporta in sicurezza le build free-threaded e sono stati aggiornati i test e la documentazione pertinenti.