Refs #26601 -- Made get_response argument required and don't accept None in middleware classes.

Per deprecation timeline.

tests/deprecation/test_middleware_mixin.py

@@ -28,14 +28,10 @@ from django.middleware.http import ConditionalGetMiddleware
 from django.middleware.locale import LocaleMiddleware
 from django.middleware.security import SecurityMiddleware
 from django.test import SimpleTestCase
-from django.utils.deprecation import MiddlewareMixin, RemovedInDjango40Warning
+from django.utils.deprecation import MiddlewareMixin
 
 
 class MiddlewareMixinTests(SimpleTestCase):
-    """
-    Deprecation warning is raised when using get_response=None.
-    """
-    msg = 'Passing None for the middleware get_response argument is deprecated.'
     middlewares = [
         AuthenticationMiddleware,
         BrokenLinkEmailsMiddleware,
@@ -58,16 +54,11 @@ class MiddlewareMixinTests(SimpleTestCase):
         XViewMiddleware,
     ]
 
-    def test_deprecation(self):
-        for middleware in self.middlewares:
-            with self.subTest(middleware=middleware):
-                with self.assertRaisesMessage(RemovedInDjango40Warning, self.msg):
-                    middleware()
-
     def test_passing_explicit_none(self):
+        msg = 'get_response must be provided.'
         for middleware in self.middlewares:
             with self.subTest(middleware=middleware):
-                with self.assertRaisesMessage(RemovedInDjango40Warning, self.msg):
+                with self.assertRaisesMessage(ValueError, msg):
                     middleware(None)
 
     def test_coroutine(self):