Use typing_extensions.Self to support older pythons in CI

tests/typecheck/managers/querysets/test_as_manager.yml

@@ -446,7 +446,7 @@
             content: |
                 from __future__ import annotations
 
-                from typing import Self
+                from typing_extensions import Self
 
                 from django.contrib.auth import get_user_model
                 from django.db import models