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

This reverts commit 1234ddf8a5c98d5d700c82e087f04725170ad581.
diff --git a/lib/py/src/protocol/TBase.py b/lib/py/src/protocol/TBase.py
index 9ae1b11..6c6ef18 100644
--- a/lib/py/src/protocol/TBase.py
+++ b/lib/py/src/protocol/TBase.py
@@ -80,3 +80,7 @@
                                       [self.__class__, self.thrift_spec])
         else:
             return iprot.readStruct(cls, cls.thrift_spec, True)
+
+
+class TFrozenExceptionBase(TFrozenBase, TExceptionBase):
+    pass