Starting with PyGObject 3.1.92 there's a module called pygtkcompat which will make it easier to port an application from PyGTK.
There are two different approaches to porting your application from PyGTK to gobject-introspection
Using a PyGTK compatibility layer (pygtkcompat)
Using a shell script which converts the callsites (pygi-convert.sh)
There are advantages and disadvantages with both:
Using pygtkcompat makes it possible to focus on porting to Gtk 3.x while not changing anything
else, users and developers can keep on using the software as before.
Using pygtkcompat allows you to support both Gtk 2.x and Gtk 3.x with the same code base
Using pygi-convert.sh you can avoid using any layers and use the supported API directly