Fixed #21430 -- Added a RuntimeWarning when unpickling Models and QuerySets from a different Django version.

Thanks FunkyBob for the suggestion, prasoon2211 for the initial patch,
and akaariai, loic, and charettes for helping in shaping the patch.

tests/model_regress/test_pickle.py

@@ -0,0 +1,53 @@
+import pickle
+import warnings
+
+from django.db import models, DJANGO_VERSION_PICKLE_KEY
+from django.test import TestCase
+from django.utils.encoding import force_text
+from django.utils.version import get_major_version, get_version
+
+
+class ModelPickleTestCase(TestCase):
+    def test_missing_django_version_unpickling(self):
+        """
+        #21430 -- Verifies a warning is raised for models that are
+        unpickled without a Django version
+        """
+        class MissingDjangoVersion(models.Model):
+            title = models.CharField(max_length=10)
+
+            def __reduce__(self):
+                reduce_list = super(MissingDjangoVersion, self).__reduce__()
+                data = reduce_list[-1]
+                del data[DJANGO_VERSION_PICKLE_KEY]
+                return reduce_list
+
+        p = MissingDjangoVersion(title="FooBar")
+        with warnings.catch_warnings(record=True) as recorded:
+            pickle.loads(pickle.dumps(p))
+            msg = force_text(recorded.pop().message)
+            self.assertEqual(msg,
+                "Pickled model instance's Django version is not specified.")
+
+    def test_unsupported_unpickle(self):
+        """
+        #21430 -- Verifies a warning is raised for models that are
+        unpickled with a different Django version than the current
+        """
+        class DifferentDjangoVersion(models.Model):
+            title = models.CharField(max_length=10)
+
+            def __reduce__(self):
+                reduce_list = super(DifferentDjangoVersion, self).__reduce__()
+                data = reduce_list[-1]
+                data[DJANGO_VERSION_PICKLE_KEY] = str(float(get_major_version()) - 0.1)
+                return reduce_list
+
+        p = DifferentDjangoVersion(title="FooBar")
+        with warnings.catch_warnings(record=True) as recorded:
+            pickle.loads(pickle.dumps(p))
+            msg = force_text(recorded.pop().message)
+            self.assertEqual(msg,
+                "Pickled model instance's Django version %s does not "
+                "match the current version %s."
+                % (str(float(get_major_version()) - 0.1), get_version()))