py/objdict: Fix popitem for ordered dicts.

The popitem method wasn't implemented for ordered dicts and would result in
an invalid state.

Fixes issue #5956.

py/map.c

@@ -177,6 +177,7 @@ mp_map_elem_t *mp_map_lookup(mp_map_t *map, mp_obj_t index, mp_map_lookup_kind_t
                     --map->used;
                     memmove(elem, elem + 1, (top - elem - 1) * sizeof(*elem));
                     // put the found element after the end so the caller can access it if needed
+                    // note: caller must NULL the value so the GC can clean up (e.g. see dict_get_helper).
                     elem = &map->table[map->used];
                     elem->key = MP_OBJ_NULL;
                     elem->value = value;