[1.6.x] Updated the bundled version of six to 1.4.0.

Backport of 42920970 from master.
[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.
Used six.moves.zip_longest, new in six 1.4.0.