Add __contains__ to EvalEnvironment

It is used in _get_on_change_args

tryton/gui/window/view_form/model/record.py

@@ -42,6 +42,9 @@ class EvalEnvironment(dict):
 
     __repr__ = __str__
 
+    def __contains__(self, item):
+        return item in self.parent.mgroup.mfields
+
 
 class ModelRecord(SignalEvent):