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

@@ -114,7 +114,7 @@ class OneToOneRegressionTests(TestCase):
         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),
@@ -235,3 +235,11 @@ class OneToOneRegressionTests(TestCase):
         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)