frame_traverse(): Use the standard Py_VISIT macro.

frame_traverse():  Use the standard Py_VISIT macro.

Py_VISIT:  cast the `op` argument to PyObject* when calling
`visit()`.  Else the caller has to pay too much attention to
this silly detail (e.g., frame_traverse needs to traverse
`struct _frame *` and `PyCodeObject *` pointers too).