Discussion of XTKS (JPX) holidays

[rsheftel]

@gerrymanoim I don't see the XKTS (JPX) calendar in your difference report, but doing a spot check the holiday 2021-09-23, the Autumnal Equinox (as shown here: https://www.tradinghours.com/exchanges/jpx/market-holidays/2021) is a holiday in pandas_market_calendars but not in trading_calendars

These holidays were provided to pandas_market_calendars in this PR: rsheftel/pandas_market_calendars#72

[gerrymanoim]

Hey @rsheftel - I actually didn't compare anything past the end of 2020, since we haven't gone through and updated all the calendars for 2021 (@jmccorriston and I have it as a todo list item).

The equinoxes are defined at https://github.com/quantopian/trading_calendars/blob/master/trading_calendars/xtks_holidays.py#L87. We can/should port over the 2021 holidays though.