Problem with update-manager: No module named 'apt_pkg' in Ubuntu 13.10, having installed Python 3.4 on /usr/local/lib

I'm rather new to Ubuntu and I'm trying to run update-manager on Ubuntu 13.10. I get:

jacopo@jacopo-laptop:~$ update-manager 
Traceback (most recent call last):
  File "/usr/bin/update-manager", line 28, in <module>
    from gi.repository import Gtk
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 27, in <module> from ._gi import _API
ImportError: No module named 'gi._gi'
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 64, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 20, in <module>
import apt
File "/usr/lib/python3/dist-packages/apt/__init__.py", line 21, in <module>
import apt_pkg
ImportError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
File "/usr/bin/update-manager", line 28, in <module>
from gi.repository import Gtk
File "/usr/lib/python3/dist-packages/gi/__init__.py", line 27, in <module>
from ._gi import _API
ImportError: No module named 'gi._gi'

As far as I have understood, this is a problem related to the fact that I should have python3.4 installed. I installed it in /usr/local/lib, but I always get the same error. In this way I can't even upgrade to Ubuntu 14.04.

Reinstall apt_pkg using:

sudo apt-get install --reinstall python3-apt

The error is primarily because of library apt_pkg.cpython-35m-x86_64-linux-gnu.so not being present in /usr/lib/python3/dist-packages.

Removing and reinstalling should help.

update-alternatives  --set python3  /usr/bin/python3.6