Fix Python GI API version warning in datastore

When the datastore process begins on Fedora 26, the import of mime.py
causes a version warning to the system journal.

```
Jun 09 10:28:01 localhost-live org.laptop.sugar.DataStore[2197]: /usr/lib/python2.7/site-packages/sugar3/mime.py:29: PyGIWarning: GdkPixbuf was imported without specifying a version first. Use gi.require_version('GdkPixbuf', '2.0') before import to ensure that the right version gets loaded.
Jun 09 10:28:01 localhost-live org.laptop.sugar.DataStore[2197]:   from gi.repository import GdkPixbuf
```

Accidental finding on #4989.

src/sugar3/mime.py

@@ -25,6 +25,9 @@ import os
 import logging
 import gettext
 
+import gi
+gi.require_version('GdkPixbuf', '2.0')
+
 from gi.repository import GLib
 from gi.repository import GdkPixbuf
 from gi.repository import Gio