Update Python version to >= 2.5

[Luca Ferretti <elle uca libero it>]

Current version is 2.4.3[1] and I know a Python update was yet discussed
in past, but please also consider:

      * libproxy (suggested for 2.26) ask for python >= 2.5
      * gobject-introspection (needed by gnome-shell) ask for python >=
        2.5
      * other modules I'm missing?

I think this is the right time to update from 2.4 to 2.5 (or 2.6
directly), or at least do it in `jhbuild bootstrap`.


[1] http://live.gnome.org/TwoPointTwentyfive/ExternalDependencies