Function PyLong_AsNativeBytes 

pub unsafe extern "C" fn PyLong_AsNativeBytes(
    v: *mut PyObject,
    buffer: *mut c_void,
    n_bytes: Py_ssize_t,
    flags: c_int,
) -> Py_ssize_t

Available on Py_3_14, or Py_3_13 and non-Py_LIMITED_API only.