Make gtk-builder-convert compatible with Python 3

Hello! Since gtk-builder-convert is distributed as part of GTK 2, would you accept this patch to make it compatible with Python 3 (and Python 2.6+ as well)? This repo seems like a good place to make the patch available for anyone who's still interested in it.

Not knowing much about GTK/glade, I smoke-tested it on .glade files I found removed here: https://bugzilla.gnome.org/show_bug.cgi?id=463010 ( https://bug463010.bugzilla-attachments.gnome.org/attachment.cgi?id=94506 ) The versions before the patch and after the patch with py2 give the same output; on py3 there is a difference in the order of tags.