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

PySlice_GetIndicesEx in _ctypes.c.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 689529a..d011f7d 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -8008,7 +8008,7 @@
         Py_UNICODE* result_buf;
         PyObject* result;
 
-        if (_PySlice_Unpack((PySliceObject *)item, &start, &stop, &step) < 0) {
+        if (_PySlice_Unpack(item, &start, &stop, &step) < 0) {
             return NULL;
         }
         slicelength = _PySlice_AdjustIndices(PyUnicode_GET_SIZE(self), &start,