Python version and jhbuild

Hey all.

Migrating Orca to Python 3 has been on the to-do list for a while now.
Upon being told that Ubuntu was hoping to have its next release be all
Python 3 (at least for their live CD), I decided to go ahead and do it.
That work has been committed to master. Furthermore, the Accessibility
Team felt that this was worth doing for all its modules. As a result,
pyatspi2 is now Python 3 compatible and Accerciser's migration is
underway (just waiting for the code review).

This change, unfortunately, makes jhbuild unhappy when building Orca
(and later Accerciser) in an environment which lacks Python 3 versions
of the build dependencies. I was hoping it would be relatively
straightforward to adjust jhbuild to handle this new situation. But the
solution didn't jump out at me, so I asked in #release-team because I
didn't want to make their lives unpleasant during next week's release.
What I was told by Walters is:

" ... anyways we don't have a way in jhbuild to express a dependency on
python3 that i'm aware of (jhbuild has always just been an out of date
snapshot of some of the dependencies of a given component)."

And Matthias said I should ask here. :)

Ideas?

Thanks!
--joanie