Return a computation instance for the given `message` and `transaction_context`

def get_computation(self,
                        message: Message,
                        transaction_context: 'BaseTransactionContext') -> 'BaseComputation':
        """
        Return a computation instance for the given `message` and `transaction_context`
        """
        if self.computation_class is None:
            raise AttributeError("No `computation_class` has been set for this State")
        else:
            computation = self.computation_class(self, message, transaction_context)
        return computation