Breaking python2.4 compatibility for bzr-2.4

[Martin Pool]

OK, so it seems like we can move to at least python2.5 for bzr2.4.
The main problem with moving to python2.6 is that is harder to
redistribute py2exe binaries on Windows; one main benefit is that it
will let us mark b'' byte strings which will help with python3.  A
side benefit of sticking with python.5 is that it will keep support
for a shipped python interpreter on old linux distributions like
Ubuntu Hardy.

<https://code.launchpad.net/~jameinel/bzr/test-traceback-767177/+merge/58505>
is blocked on deciding about this.  It declares a requirement for
python2.5 but it seems like the actual code will work on 2.5.

So I think for now we should go to just 2.5, and then later, perhaps
driven by python3, we'll can look at requiring python2.6.  Perhaps
even before the bzr2.4 final release (~August).  Perhaps the Windows
distribution situation will have improved.

Martin