ListStore needs have a str or GOBject otherwise TraceBack

if six.text_type is used then following traceback is seen.
  File "/home/phracek/work/programming/devassistant/devassistant/command_helpers.py", line 414, in ask_for_package_list_confirm
    liststore = Gtk.ListStore(six.text_type)
  File "/usr/lib64/python2.7/site-packages/gi/overrides/Gtk.py", line 932, in __init__
    self.set_column_types(column_types)
TypeError: Item 0: Must be gobject.GType, not type

After changing back to str all works fine.
It was caused by commit (https://github.com/bkabrda/devassistant/commit/90f0929524365ba7d088de7b00d3d2076efc2a16)