Workaround for parity https://github.com/paritytech/parity-ethereum/issues/9707
        In parity doing any call() with the 'pending' block no longer falls back
        to the latest if no pending block is found but throws a mistaken error.
        Until that bug is fixed we need to enforce special behaviour for parity
        and use the latest block for checking.

def get_checking_block(self):
        """Workaround for parity https://github.com/paritytech/parity-ethereum/issues/9707
        In parity doing any call() with the 'pending' block no longer falls back
        to the latest if no pending block is found but throws a mistaken error.
        Until that bug is fixed we need to enforce special behaviour for parity
        and use the latest block for checking.
        """
        checking_block = 'pending'
        if self.eth_node is constants.EthClient.PARITY:
            checking_block = 'latest'
        return checking_block