Python 3 support

[Julien Rioux]

Hi,

Our python scripts currently require a python2 interpreter with version 
at least 2.4, but by bumping the requirement to 2.6 we could start 
introducing
from __future__ import print_function
which will be one of the major step towards supporting both python2 and 
python3 interpreters with a single codebase.

One of the big hurdle to bumping the python version requirement is our 
own cross-platform build system GUB which provides 2.4, but last summer 
I was able to build lilypond with an updated GUB, based on some 
experimental branch by Jan where I added a few commits. You can find the 
patchset here: https://github.com/gperciva/gub/pull/6

I think the following would be a good 1-2 punch approach for the current 
development cycle: First, upgrade python in GUB and make sure we can 
build current dev and stable branches of lilypond from it. Then bump the 
python requirement in the dev branch and start migrating to a codebase 
that supports python 2.6+ and python 3+

Thoughts? Objections?

Regards,
Julien