Complex sentence: PyGTK will be phased out with the transition to GTK + version 3 and be replaced with PyGObject, which uses GObject introspection to will be used to generate bindings for Python and other languages on the fly.
Split into simpler sentences: PyGTK will be phased out with the transition to GTK + version 3. Instead, GObject introspection will be used to generate bindings for Python and other languages on the fly.