gi repository problem

Question

Quando tento executar software-center , recebo esta mensagem

Traceback (most recent call last):
File "/usr/bin/software-center", line 25, in <module>
    from gi.repository import Gtk, GObject
ImportError: No module named repository

Mas eu instalei gi por

apt-get install python3-gi 

mas diz: python3-gi is already the newest version.