gh-111178: Fix function signature for test_threading

[vstinner]

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178

[picnixz]

You can remove the explicit cast to _Py_once_fn_t:

    _PyOnceFlag_CallOnce(&self->once, (_Py_once_fn_t *)force_done, self);