Free-threaded CPython support

[aslangencer]

I attempted to compile av for the free-threaded CPython (cpython-3.13.0+freethreaded-linux-x86_64-gnu) both with and without the LDFLAG --no-build-isolation but both attempts failed with the following error:

clang -pthread -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -fPIC -O0 -Wno-incompatible-pointer-types -Wno-unreachable-code -fPIC -g -Iav/filter -I/usr/include/x86_64-linux-gnu -I/dev/shm/new_proj/.venv/include -I/home/testuser/.local/share/uv/python/cpython-3.13.0+freethreaded-linux-x86_64-gnu/include/python3.13t -c src/av/filter/loudnorm.c -o build/temp.linux-x86_64-cpython-313t/src/av/filter/loudnorm.o
src/av/filter/loudnorm.c:2751:80: error: unknown type name '__pyx_vectorcallfunc'; did you mean 'vectorcallfunc'?
 2751 | static CYTHON_INLINE PyObject *__Pyx_PyVectorcall_FastCallDict(PyObject *func, __pyx_vectorcallfunc vc, PyObject *const *args, size_t nargs, PyObject *kw);
      |                                                                                ^~~~~~~~~~~~~~~~~~~~
      |                                                                                vectorcallfunc

I have numpy 2.1.3 in this venv. Any ideas how to resolve this error?
Any suggestions?

[WyattBlue]

I don't think Cython supported free thread mode yet