Function synthesized_traverse 

pub unsafe extern "C" fn synthesized_traverse<T>(
    slf: *mut PyObject,
    visit: visitproc,
    arg: *mut c_void,
) -> c_int
where
    T: PyClass,

tp_traverse for a #[pyclass] which defines no __traverse__ of its own: visits the base type and the instance __dict__ (if it is a #[pyclass(dict)]).