bug#25457: assword: gui: Namespace Gtk not available

[Ivan Vilata i Balaguer]

Hi, when running ``assword gui`` from Assword 0.10 I get the following error:

```
Traceback (most recent call last):
  File 
"/gnu/store/wicbdk90fihvvc2zvbvn1wc7jfv8ypbi-assword-0.10/bin/.assword-real", 
line 9, in <module>
    load_entry_point('assword==0.10', 'console_scripts', 'assword')()
  File 
"/gnu/store/wicbdk90fihvvc2zvbvn1wc7jfv8ypbi-assword-0.10/lib/python3.5/site-packages/assword/__main__.py",
 line 553, in main
    func(args)
  File 
"/gnu/store/wicbdk90fihvvc2zvbvn1wc7jfv8ypbi-assword-0.10/lib/python3.5/site-packages/assword/__main__.py",
 line 327, in gui
    from assword.gui import Gui
  File 
"/gnu/store/wicbdk90fihvvc2zvbvn1wc7jfv8ypbi-assword-0.10/lib/python3.5/site-packages/assword/gui.py",
 line 2, in <module>
    gi.require_version('Gtk', '3.0')
  File 
"/gnu/store/hii5dmg5qyawx6a883203dcxhbagmgrd-python-pygobject-3.20.0/lib/python3.5/site-packages/gi/__init__.py",
 line 102, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace Gtk not available
```

Other commands not using the GUI work ok.  The error does not show with Assword 
0.8.

```
$ guix package -I | grep assword
assword 0.10    out     /gnu/store/wicbdk90fihvvc2zvbvn1wc7jfv8ypbi-assword-0.10
```

Thank you!

-- 
Ivan Vilata i Balaguer -- https://elvil.net/