Specify library version prior to import

- web activities caused two warnings every time they were started,

- warnings cost disk write wait time.

src/sugar3/activity/webactivity.py

@@ -19,6 +19,10 @@ import json
 import os
 import logging
 
+import gi
+gi.require_version('Gtk', '3.0')
+gi.require_version('WebKit2', '4.0')
+
 from gi.repository import Gdk
 from gi.repository import Gio
 from gi.repository import WebKit2