Import pyson locally to not break installation

issue2705
review450001
(transplanted from 433368c5f09898fced213c19da8c82741d9caab0)

proteus/__init__.py

@@ -10,7 +10,6 @@ import threading
 import datetime
 from decimal import Decimal
 import proteus.config
-from proteus.pyson import PYSONDecoder
 
 _MODELS = threading.local()
 
@@ -707,6 +706,8 @@ class Model(object):
 
     def _on_change(self, name):
         'Call on_change for field'
+        # Import locally to not break installation
+        from proteus.pyson import PYSONDecoder
         definition = self._fields[name]
         if definition.get('on_change'):
             if isinstance(definition['on_change'], basestring):