bug#40834: system-config-printer fails while starting

[Jonathan Brielmaier]

system-config-printer fails to start on current master as well as on 
core-updates.

```
$ system-config-printer
Traceback (most recent call last):
  File 
"/gnu/store/y77skk51wc3j1bikfdyc0mk5j62fj05s-system-config-printer-1.5.12/share/system-config-printer/system-config-printer.py",
 line 40, in <module>
    gi.require_version('Gdk', '3.0')
  File 
"/gnu/store/y4qwsw06ynvw3fjk2lrspp89mfmjzj5i-python-pygobject-3.28.3/lib/python3.7/site-packages/gi/__init__.py",
 line 130, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace Gdk not available
```

While python-pygobject provides a Gdk.py:
/gnu/store/y4qwsw06ynvw3fjk2lrspp89mfmjzj5i-python-pygobject-3.28.3/lib/python3.7/site-packages/gi/overrides/Gdk.py

I guess this is some issue with (python) paths