[pygobject] examples/demo: Use require_version to force gtk3

[Christoph Reiter <creiter src gnome org>]

commit 7df001655da6bd31c023c010a66e272f468a762a
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Mon Nov 30 18:31:22 2020 +0100

    examples/demo: Use require_version to force gtk3
    
    The examples don't work with gtk4 so set the proper version

 examples/demo/demo.py | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/examples/demo/demo.py b/examples/demo/demo.py
index e06fd99d..6e269943 100755
--- a/examples/demo/demo.py
+++ b/examples/demo/demo.py
@@ -25,6 +25,13 @@ import os
 import sys
 import textwrap
 
+import gi
+gi.require_version('Gtk', '3.0')
+gi.require_version('Pango', '1.0')
+gi.require_version('PangoCairo', '1.0')
+gi.require_version('GdkPixbuf', '2.0')
+gi.require_version('GtkSource', '4')
+
 from gi.repository import GLib, GObject, Pango, GdkPixbuf, Gtk, Gio
 
 try: