Revert "Revert "THRIFT-4002: Make generated exception classes immutable by default""

This reverts commit 1234ddf8a5c98d5d700c82e087f04725170ad581.
diff --git a/test/py.tornado/test_suite.py b/test/py.tornado/test_suite.py
index 447fde6..0ee0a9b 100755
--- a/test/py.tornado/test_suite.py
+++ b/test/py.tornado/test_suite.py
@@ -82,10 +82,7 @@
 
     def testException(self, s):
         if s == 'Xception':
-            x = Xception()
-            x.errorCode = 1001
-            x.message = s
-            raise x
+            raise Xception(1001, s)
         elif s == 'throw_undeclared':
             raise ValueError('testing undeclared exception')