Use PyList_CheckExact and PyTuple_CheckExact for checking whether
PySequence_Fast needs to do anything siginificant.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 9940fd3..4f8dd3e 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -1436,7 +1436,7 @@
 	if (v == NULL)
 		return null_error();
 
-	if (PyList_Check(v) || PyTuple_Check(v)) {
+	if (PyList_CheckExact(v) || PyTuple_CheckExact(v)) {
 		Py_INCREF(v);
 		return v;
 	}