[attributs] assert value is not only unicode but can also be of the specified python type

attributs.py

@@ -211,7 +211,7 @@ class Attr(object):
         """Crée un nouvel objet représentant un attribut."""
         self.value = None
         self.conn = conn
-        assert isinstance(val, unicode)
+        assert isinstance(val, unicode) or (self.python_type and isinstance(val, self.python_type))
         self.parent = Parent
         self.parse_value(val)