[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/utils_tests/test_datastructures.py

@@ -50,7 +50,7 @@ class SortedDictTests(SimpleTestCase):
         self.d2[7] = 'lucky number 7'
         self.assertEqual(list(six.iterkeys(self.d2)), [1, 9, 0, 7])
 
-    if not six.PY3:
+    if six.PY2:
         def test_change_keys(self):
             """
             Changing the keys won't do anything, it's only a copy of the