py: Replace mp_const_stop_iteration object with MP_OBJ_NULL.
diff --git a/py/objtuple.c b/py/objtuple.c
index 3378b4e..68f5abe 100644
--- a/py/objtuple.c
+++ b/py/objtuple.c
@@ -53,7 +53,7 @@
 
             mp_obj_t iterable = rt_getiter(args[0]);
             mp_obj_t item;
-            while ((item = rt_iternext(iterable)) != mp_const_stop_iteration) {
+            while ((item = rt_iternext(iterable)) != MP_OBJ_NULL) {
                 if (len >= alloc) {
                     items = m_renew(mp_obj_t, items, alloc, alloc * 2);
                     alloc *= 2;
@@ -245,7 +245,7 @@
         self->cur += 1;
         return o_out;
     } else {
-        return mp_const_stop_iteration;
+        return MP_OBJ_NULL;
     }
 }