test_classes: Use mock.patch in tornado_redirected_to_list.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/test_classes.py

@@ -98,7 +98,6 @@ from zerver.models import (
     get_user_by_delivery_email,
 )
 from zerver.openapi.openapi import validate_against_openapi_schema, validate_request
-from zerver.tornado import django_api as django_tornado_api
 from zerver.tornado.event_queue import clear_client_event_queues_for_testing
 
 if settings.ZILENCER_ENABLED:
@@ -1314,15 +1313,15 @@ Output:
         self, lst: List[Mapping[str, Any]], expected_num_events: int
     ) -> Iterator[None]:
         lst.clear()
-        real_event_queue_process_notification = django_tornado_api.process_notification
 
         # process_notification takes a single parameter called 'notice'.
         # lst.append takes a single argument called 'object'.
         # Some code might call process_notification using keyword arguments,
         # so mypy doesn't allow assigning lst.append to process_notification
         # So explicitly change parameter name to 'notice' to work around this problem
-        django_tornado_api.process_notification = lambda notice: lst.append(notice)
-
+        with mock.patch(
+            "zerver.tornado.django_api.process_notification", lambda notice: lst.append(notice)
+        ):
             # Some `send_event` calls need to be executed only after the current transaction
             # commits (using `on_commit` hooks). Because the transaction in Django tests never
             # commits (rather, gets rolled back after the test completes), such events would
@@ -1331,8 +1330,6 @@ Output:
             with self.captureOnCommitCallbacks(execute=True):
                 yield
 
-        django_tornado_api.process_notification = real_event_queue_process_notification
-
         self.assert_length(lst, expected_num_events)
 
     def create_user_notifications_data_object(