bpo-30074: Fix compile warnings of _PySlice_Unpack and convert missed (#1154)

PySlice_GetIndicesEx in _ctypes.c.
diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
index 808156f..a61c8aa 100644
--- a/Objects/tupleobject.c
+++ b/Objects/tupleobject.c
@@ -715,7 +715,7 @@
         PyObject* it;
         PyObject **src, **dest;
 
-        if (_PySlice_Unpack((PySliceObject *)item, &start, &stop, &step) < 0) {
+        if (_PySlice_Unpack(item, &start, &stop, &step) < 0) {
             return NULL;
         }
         slicelength = _PySlice_AdjustIndices(PyTuple_GET_SIZE(self), &start,