Add temporary holiday to xkrx exchange

Author: sjquant

tests/resources/xkrx.csv

@@ -8536,7 +8536,6 @@
 2020-08-12,2020-08-12 00:01:00,2020-08-12 06:30:00
 2020-08-13,2020-08-13 00:01:00,2020-08-13 06:30:00
 2020-08-14,2020-08-14 00:01:00,2020-08-14 06:30:00
-2020-08-17,2020-08-17 00:01:00,2020-08-17 06:30:00
 2020-08-18,2020-08-18 00:01:00,2020-08-18 06:30:00
 2020-08-19,2020-08-19 00:01:00,2020-08-19 06:30:00
 2020-08-20,2020-08-20 00:01:00,2020-08-20 06:30:00

trading_calendars/exchange_calendar_xkrx.py

@@ -517,6 +517,7 @@
     "2020-04-30",
     "2020-05-01",
     "2020-05-05",
+    "2020-08-17",
     "2020-09-30",
     "2020-10-01",
     "2020-10-02",