Comment 7 for bug 1840677

Note: I'm not entirely sure why ${python3:Depends} didn't pick it up.
It does so for
virtManager/details/viewers.py:17: gi.require_version('SpiceClientGtk', '3.0')
  => gir1.2-spiceclientgtk-3.0
virtManager/details/serialcon.py:16: gi.require_version("Vte", "2.91")
  => gir1.2-vte-2.91
virtManager/details/viewers.py:14:gi.require_version('GtkVnc', '2.0')
  => gir1.2-gtk-vnc-2.0
virt-manager:61: gi.require_version('Gtk', '3.0')
  => gir1.2-gtk-3.0
virt-manager:16:gi.require_version('LibvirtGLib', '1.0')
  => gir1.2-libvirt-glib-1.0

Just not:
  gir1.2-gtksource-4

Ah no, there is another one
  gir1.2-libosinfo-1.0
back from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=796723

Ok, lets add the dependency at the same level - that seems the right place.