cannot import name Gio (lp:#1750168)

[Original report](https://bugs.launchpad.net/bugs/1750168) created by **Dan Jaenecke (wonk042)**
```
I am using duplicity via deja-dup to create backups; but recently it
stopped working, stating "BackendException: Hintergrundprogramm konnte
nicht initialisiert werden: cannot import name Gio" (sorry for the german
error messages, unfortunately I failed to switch to english messages).

The command causing this error (reduced to the minimum):
duplicity --verbosity=9 --gio collection-status file:///tmp
Achtung: Parameter --gio ist veraltet und wird in einer der nächsten
Versionen entfernt.
Bitte verwenden Sie Standarddateinamen.
Folgendes Archivverzeichnis wird benutzt:
$HOME/.cache/duplicity/c2731c0788339744944161fd8afb74dd
Folgender Sicherungsname wird benutzt: c2731c0788339744944161fd8afb74dd
Import von duplicity.backends.azurebackend Succeeded
Import von duplicity.backends.b2backend Succeeded
Import von duplicity.backends.botobackend Succeeded
Import von duplicity.backends.cfbackend Succeeded
Import von duplicity.backends.copycombackend Succeeded
Import von duplicity.backends.dpbxbackend Succeeded
Import von duplicity.backends.gdocsbackend Succeeded
Import von duplicity.backends.giobackend Succeeded
Import von duplicity.backends.hsibackend Succeeded
Import von duplicity.backends.hubicbackend Succeeded
Import von duplicity.backends.imapbackend Succeeded
Import von duplicity.backends.lftpbackend Succeeded
Import von duplicity.backends.localbackend Succeeded
Import von duplicity.backends.megabackend Succeeded
Import von duplicity.backends.multibackend Succeeded
Import von duplicity.backends.ncftpbackend Succeeded
Import von duplicity.backends.onedrivebackend Succeeded
Import von duplicity.backends.par2backend Succeeded
Import von duplicity.backends.pydrivebackend Succeeded
Import von duplicity.backends.rsyncbackend Succeeded
Import von duplicity.backends.ssh_paramiko_backend Succeeded
Import von duplicity.backends.ssh_pexpect_backend Succeeded
Import von duplicity.backends.swiftbackend Succeeded
Import von duplicity.backends.sxbackend Succeeded
Import von duplicity.backends.tahoebackend Succeeded
Import von duplicity.backends.webdavbackend Succeeded
Temporäres Verzeichnis /tmp/duplicity-EguFuP-tempdir wird benutzt
Fehlerdetail des Hintergrundprogramms: Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1532, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1526, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1364, in main
    action = commandline.ProcessCommandLine(sys.argv[1:])
  File "/usr/lib/python2.7/dist-packages/duplicity/commandline.py", line
1108, in ProcessCommandLine
    globals.backend = backend.get_backend(args[0])
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 223,
in get_backend
    obj = get_backend_object(url_string)
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 211,
in get_backend_object
    raise BackendException(_("Could not initialize backend: %s") %
str(sys.exc_info()[1]))
BackendException: Hintergrundprogramm konnte nicht initialisiert werden:
cannot import name Gio

Removing --gio from the command fixes the problem, but since the command is
created by deja-dup I have no idea how to get rid of that option.

A possible solutions I found was that the package python-gi might be
missing; it was already installed, and even a
$ apt install --reinstall python-gi
did not help.
Also as suggested in Question #664503 I tried installing python-gobject-2 /
python-gobject-2-dev, which did not help either

My OS: Ubuntu 16.04.3 LTS
Version of duplicity: 0.7.06
Python 2.7.12
```

Story points: 6.0