py: De-optimise some uses of mp_getiter, so they don't use the C stack.

In these cases the heap is anyway used to create a new object so no real
need to use the C stack for iterating.  It saves a few bytes of code size.
diff --git a/py/objlist.c b/py/objlist.c
index 28da109..55ee191 100644
--- a/py/objlist.c
+++ b/py/objlist.c
@@ -60,8 +60,7 @@
 }
 
 STATIC mp_obj_t list_extend_from_iter(mp_obj_t list, mp_obj_t iterable) {
-    mp_obj_iter_buf_t iter_buf;
-    mp_obj_t iter = mp_getiter(iterable, &iter_buf);
+    mp_obj_t iter = mp_getiter(iterable, NULL);
     mp_obj_t item;
     while ((item = mp_iternext(iter)) != MP_OBJ_STOP_ITERATION) {
         mp_obj_list_append(list, item);