Breaking python2.4 compatibility for bzr-2.4

[Martin Pool]

OK, let's do it:

 * bzr 2.4 will require 2.6
 * we can work towards python3 compatibility
 * bzr 2.3 will stay on python2.4 (of course) and if we add a new
default format during its life time we'll backport support
 * we welcome people nominating particular bugs they think ought to be
fixed in a stable series

I think John (?) has a branch that bumps up the python requirement.
Can you please repropose and land it?

Martin