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

This reverts commit 1234ddf8a5c98d5d700c82e087f04725170ad581.
diff --git a/test/py.twisted/test_suite.py b/test/py.twisted/test_suite.py
index 02eb7f1..6e04493 100755
--- a/test/py.twisted/test_suite.py
+++ b/test/py.twisted/test_suite.py
@@ -76,10 +76,7 @@
 
     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("foo")