Message 204028 - Python tracker

Yeah, I argued myself into realising EXTENDED_ARG just needed to be
mentioned in the function docs, but didn't go back and fix my opening
paragraph.

The fact dis uses an arg of None (rather than zero) to indicate "no
argument" means I think the extra layer of wrapping in the pure Python
module is needed prior to 3.4 rc1, but we can live without it for beta 1.