Add a final permutation step to the tuple hash function.

Prevents a collision pattern that occurs with nested tuples.
(Yitz Gale provided code that repeatably demonstrated the weakness.)

Objects/tupleobject.c

@@ -280,6 +280,7 @@ tuplehash(PyTupleObject *v)
 		x = (x ^ y) * mult;
 		mult += 82520L + len + len;
 	}
+	x += 97531L;
 	if (x == -1)
 		x = -2;
 	return x;