Get earliest valid open timestamp from Binance

        :param symbol: Name of symbol pair e.g BNBBTC
        :type symbol: str
        :param interval: Binance Kline interval
        :type interval: str

        :return: first valid timestamp

def _get_earliest_valid_timestamp(self, symbol, interval):
        """Get earliest valid open timestamp from Binance

        :param symbol: Name of symbol pair e.g BNBBTC
        :type symbol: str
        :param interval: Binance Kline interval
        :type interval: str

        :return: first valid timestamp

        """
        kline = self.get_klines(
            symbol=symbol,
            interval=interval,
            limit=1,
            startTime=0,
            endTime=None
        )
        return kline[0][0]