GTK segfault with GTK3 and mpl_toolkits

Something goes wrong when using a GTK3 backend and the mpl_toolkits. See this very minimal testcase:

```python
>>> from gi.repository import Gtk
>>> import mpl_toolkits.axisartist
/usr/lib/python2.7/dist-packages/gobject/constants.py:24: Warning: g_boxed_type_register_static: assertion `g_type_from_name (name) == 0' failed
  import gobject._gobject
/usr/lib/python2.7/dist-packages/gtk-2.0/gtk/__init__.py:40: Warning: specified class size for type `PyGtkGenericCellRenderer' is smaller than the parent type's `GtkCellRenderer' class size
  from gtk import _gtk
/usr/lib/python2.7/dist-packages/gtk-2.0/gtk/__init__.py:40: Warning: g_type_get_qdata: assertion `node != NULL' failed
  from gtk import _gtk
Segmentatiefout (geheugendump gemaakt)
```

Most likely, something imports the old GTK2 bindings, run following for the same error:

```python
>>> from gi.repository import Gtk
>>> import gtk
```

Grepping the sourcecode gives mpl_toolkits/gtktools.py where it is used, but I can't even find where it is imported.