Fix Python tutorials to address THRIFT-4002

$ make tutorialserver
/usr/bin/python PythonServer.py
Starting the server...
ping()
add(1,1)
calculate(1, Work(comment=None, num1=1, num2=0, op=4))
ERROR:root:Unexpected exception in handler
Traceback (most recent call last):
  File "gen-py/tutorial/Calculator.py", line 274, in process_calculate
    result.success = self._handler.calculate(args.logid, args.w)
  File "PythonServer.py", line 61, in calculate
    x.whatOp = work.op
  File "gen-py/tutorial/ttypes.py", line 160, in __setattr__
    raise TypeError("can't modify immutable instance")
TypeError: can't modify immutable instance

$ make tutorialclient
/usr/bin/python PythonClient.py
ping()
1+1=2
Internal error