Changed the Debian package dependency to python3. This required the gmate.py script to change to using python3.
Pruned the Groove language spec, as it conflicts with GtkSourceView.
Added source/format file for the Debian package, as per recommendations (used 3.0 native).
Corrected Standards-Version field in Debian package (used 4.1.4).
Increased debhelper dependency to version 9 (up from version 7).