[1.6.x] Replaced "not PY3" by "PY2", new in six 1.4.0.

Conflicts:
	django/db/backends/oracle/base.py
	django/db/backends/sqlite3/base.py
	django/db/models/base.py

Backport of 365c3e8b from master.

tests/basic/tests.py

@@ -350,7 +350,7 @@ class ModelTest(TestCase):
              "<Article: Third article>"])
 
         # Slicing works with longs (Python 2 only -- Python 3 doesn't have longs).
-        if not six.PY3:
+        if six.PY2:
             self.assertEqual(Article.objects.all()[long(0)], a)
             self.assertQuerysetEqual(Article.objects.all()[long(1):long(3)],
                 ["<Article: Second article>", "<Article: Third article>"])