Fixed #21188 -- Introduced subclasses for to-be-removed-in-django-XX warnings

Thanks Anssi Kääriäinen for the idea and Simon Charette for the
review.

tests/model_forms/tests.py

@@ -13,8 +13,9 @@ from django.core.validators import ValidationError
 from django.db import connection
 from django.db.models.query import EmptyQuerySet
 from django.forms.models import model_to_dict
-from django.utils._os import upath
 from django.test import TestCase, skipUnlessDBFeature
+from django.utils.deprecation import RemovedInDjango18Warning
+from django.utils._os import upath
 from django.utils import six
 
 from .models import (Article, ArticleStatus, BetterWriter, BigInt, Book,
@@ -265,7 +266,7 @@ class ModelFormBaseTest(TestCase):
 
     def test_missing_fields_attribute(self):
         with warnings.catch_warnings(record=True):
-            warnings.simplefilter("always", DeprecationWarning)
+            warnings.simplefilter("always", RemovedInDjango18Warning)
 
             class MissingFieldsForm(forms.ModelForm):
                 class Meta:
@@ -275,7 +276,7 @@ class ModelFormBaseTest(TestCase):
         # if a warning has been seen already, the catch_warnings won't
         # have recorded it. The following line therefore will not work reliably:
 
-        # self.assertEqual(w[0].category, DeprecationWarning)
+        # self.assertEqual(w[0].category, RemovedInDjango18Warning)
 
         # Until end of the deprecation cycle, should still create the
         # form as before: