I believe commit e44e848415eff3e4dab69e990adfc447e79143f8 which introduced GTK3 notification support broke GTK2 notification support fallback. The problem was not present in 0.5.0.4
I believe the problem lies in this import construct in pyhoca/wxgui/notify.py which will fail the same way even when put alone in a file
from gi.repository import Notify as _Notify
import pynotify as _Notify
I think the problem is that even if the GTK3 Notify module is not available, many GTK3 libraries get loaded and then the pynotify import fails with
ImportError: could not import gobject (could not find _PyGObject_API object)
This is the same exception I see when running pyhoca-gui.
I think some more sophisticated GTK version selection mechanism is needed, one that would not start loading conflicting libraries.
As a workaround for now I just removed that GTK3 import on my system.
My system is Debian 8.3 Jessie with the following (possibly relevant) gtk packages installed
dpkg -l | grep '^.i.*gtk.*'
ii libavahi-ui-gtk3-0:amd64 0.6.31-5 amd64 Avahi GTK+ User interface library for GTK3
ii libcanberra-gtk3-0:amd64 0.30-2.1 amd64 GTK+ 3.0 helper for playing widget event sounds with libcanberra
ii libcanberra-gtk3-module:amd64 0.30-2.1 amd64 translates GTK3 widgets signals to event sounds
ii libdbusmenu-gtk3-4:amd64 12.10.2-1 amd64 library for passing menus over DBus - GTK+ version
ii libgirara-gtk3-1:amd64 0.2.3-1 amd64 library for minimalistic user interfaces (shared libraries, GTK+ 3)
ii libgtk2-ex-podviewer-perl 0.18-1 all Perl Gtk2 widget for displaying Plain Old Documentation (POD)
ii libgtk2-ex-simple-list-perl 0.50-2 all simple interface to Gtk2's complex MVC list widget
ii libgtk2-imageview-perl 0.05-2+b1 amd64 Perl bindings for the GtkImageView image viewer widget
ii libgtk2-perl 2:1.2492-4 amd64 Perl interface to the 2.x series of the Gimp Toolkit library
ii libgtk2.0-0:amd64 2.24.25-3 amd64 GTK+ graphical user interface library
ii libgtk2.0-bin 2.24.25-3 amd64 programs for the GTK+ graphical user interface library
ii libgtk2.0-common 2.24.25-3 all common files for the GTK+ graphical user interface library
ii libwxgtk3.0-0:amd64 3.0.2-1+b1 amd64 wxWidgets Cross-platform C++ GUI toolkit (GTK+ runtime
ii pinentry-gtk2 0.8.3-2 amd64 GTK+-2-based PIN or pass-phrase entry dialog for GnuPG
ii python-gtk2 2.24.0-4 amd64 Python bindings for the GTK+ widget set
ii python-wxgtk3.0 184.108.40.206+dfsg-2 amd64 Python interface to the wxWidgets Cross-platform C++ GUI toolkit