Fixed #29708 -- Deprecated PickleSerializer.

tests/sessions_tests/tests.py

@@ -7,6 +7,7 @@ import unittest
 from datetime import timedelta
 from http import cookies
 from pathlib import Path
+from unittest import mock
 
 from django.conf import settings
 from django.contrib.sessions.backends.base import UpdateError
@@ -24,9 +25,7 @@ from django.contrib.sessions.exceptions import (
 )
 from django.contrib.sessions.middleware import SessionMiddleware
 from django.contrib.sessions.models import Session
-from django.contrib.sessions.serializers import (
-    JSONSerializer, PickleSerializer,
-)
+from django.contrib.sessions.serializers import JSONSerializer
 from django.core import management
 from django.core.cache import caches
 from django.core.cache.backends.base import InvalidCacheBackendError
@@ -880,9 +879,8 @@ class CookieSessionTests(SessionTestsMixin, SimpleTestCase):
         # by creating a new session
         self.assertEqual(self.session.serializer, JSONSerializer)
         self.session.save()
-
-        self.session.serializer = PickleSerializer
-        self.session.load()
+        with mock.patch('django.core.signing.loads', side_effect=ValueError):
+            self.session.load()
 
     @unittest.skip("Cookie backend doesn't have an external store to create records in.")
     def test_session_load_does_not_create_record(self):