Gtk justification doesn't work

[rebelxt]

Here is a very small demo:

#!/usr/bin/env python3

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk

dialog = Gtk.Dialog(title="Dialog", buttons=(Gtk.STOCK_OK, Gtk.ResponseType.OK))
dialog.set_default_size(400, -1)
box = dialog.get_content_area()

label = Gtk.Label("Label 1")
label.set_justify(Gtk.Justification.RIGHT)
box.pack_start(label, True, True, 0)
label = Gtk.Label("Label 2")
label.set_justify(Gtk.Justification.LEFT)
box.pack_start(label, True, False, 0)
label = Gtk.Label("Label 3")
label.set_justify(Gtk.Justification.RIGHT)
box.pack_start(label, False, True, 0)
label = Gtk.Label("Label 4")
label.set_justify(Gtk.Justification.LEFT)
box.pack_start(label, False, False, 0)

dialog.show_all()
response = dialog.run()
dialog.destroy()