code: def QA_util_get_pre_trade_date(cursor_date, n=1):
      
  
      cursor_date = QA_util_format_date2str(cursor_date)
      if cursor_date in trade_date_sse:
          return QA_util_date_gap(cursor_date, n, "lt")
      real_aft_trade_date = QA_util_get_real_date(cursor_date)
      return QA_util_date_gap(real_aft_trade_date, n, "lt")
docs: 得到前 n 个交易日 (不包含当前交易日)
      :param date:
      :param n: