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

This reverts commit 1234ddf8a5c98d5d700c82e087f04725170ad581.
diff --git a/lib/py/src/protocol/TProtocol.py b/lib/py/src/protocol/TProtocol.py
index 3456e8f..339a283 100644
--- a/lib/py/src/protocol/TProtocol.py
+++ b/lib/py/src/protocol/TProtocol.py
@@ -303,8 +303,14 @@
 
     def readContainerStruct(self, spec):
         (obj_class, obj_spec) = spec
-        obj = obj_class()
-        obj.read(self)
+
+        # If obj_class.read is a classmethod (e.g. in frozen structs),
+        # call it as such.
+        if getattr(obj_class.read, '__self__', None) is obj_class:
+            obj = obj_class.read(self)
+        else:
+            obj = obj_class()
+            obj.read(self)
         return obj
 
     def readContainerMap(self, spec):