Fixed #14043 -- Made sure nullable o2o delete works as expected

There was an old complaint about nullable one-to-one field cascading
even when the o2o field was saved to None value before the deletion.
Added an test to verify this doesn't happen.

Also some PEP 8 cleanup.

tests/one_to_one_regress/tests.py

@@ -25,7 +25,7 @@ def test_reverse_relationship_cache_cascade(self):
         # The bug in #9023: if you access the one-to-one relation *before*
         # setting to None and deleting, the cascade happens anyway.
         self.p1.undergroundbar
-        bar.place.name='foo'
+        bar.place.name = 'foo'
         bar.place = None
         bar.save()
         self.p1.delete()
@@ -40,12 +40,12 @@ def test_create_models_m2m(self):
         Check that we create models via the m2m relation if the remote model
         has a OneToOneField.
         """
-        f = Favorites(name = 'Fred')
+        f = Favorites(name='Fred')
         f.save()
         f.restaurants = [self.r1]
         self.assertQuerysetEqual(
-                f.restaurants.all(),
-                ['<Restaurant: Demon Dogs the restaurant>']
+            f.restaurants.all(),
+            ['<Restaurant: Demon Dogs the restaurant>']
         )
 
     def test_reverse_object_cache(self):
@@ -114,23 +114,23 @@ def test_filter_one_to_one_relations(self):
         misbehaving. We test both (primary_key=True & False) cases here to
         prevent any reappearance of the problem.
         """
-        t = Target.objects.create()
+        Target.objects.create()
 
         self.assertQuerysetEqual(
-                Target.objects.filter(pointer=None),
-                ['<Target: Target object>']
+            Target.objects.filter(pointer=None),
+            ['<Target: Target object>']
         )
         self.assertQuerysetEqual(
-                Target.objects.exclude(pointer=None),
-                []
+            Target.objects.exclude(pointer=None),
+            []
         )
         self.assertQuerysetEqual(
-                Target.objects.filter(pointer2=None),
-                ['<Target: Target object>']
+            Target.objects.filter(pointer2=None),
+            ['<Target: Target object>']
         )
         self.assertQuerysetEqual(
-                Target.objects.exclude(pointer2=None),
-                []
+            Target.objects.exclude(pointer2=None),
+            []
         )
 
     def test_reverse_object_does_not_exist_cache(self):
@@ -235,3 +235,11 @@ def test_set_reverse_on_unsaved_object(self):
         b = UndergroundBar.objects.create()
         with self.assertNumQueries(0), self.assertRaises(ValueError):
             p.undergroundbar = b
+
+    def test_nullable_o2o_delete(self):
+        u = UndergroundBar.objects.create(place=self.p1)
+        u.place_id = None
+        u.save()
+        self.p1.delete()
+        self.assertTrue(UndergroundBar.objects.filter(pk=u.pk).exists())
+        self.assertIsNone(UndergroundBar.objects.get(pk=u.pk).place)