[BUG] Add XWBO special close days on the last trading day of the year since 2017.

[jenskeiner]

This PR is to add a special close day for XWBO on the last trading day of the year. This is similar to XETR, with minor differences:

• This was introduced in 2017; see https://www.wienerborse.at/en/news/vienna-stock-exchange-news/modified-trading-hours-on-last-trading-day-29-december-2017/
• The closing auction for equities starts at 14:15 CET.

Notes:

• I've put a link to the search form on XWBO's website into the code which can be used to pull up all the official announcements regarding the last trading day of the year. Can be used to check past and future behaviour.
• The closing time is the time the closing auction for equities starts. I think this is in line with other exchanges.
• Bonds e.g. close a bit later, so this is not reflected.

BTW: There seems to be a bug, possibly in the base TradingCalendar class. When I amended the XWBO CSV file with the remaining handful of dates for 2020, some tests failed since the calendar apparently didn't see 2020-12-30 as a special close day (which it was since it was the last trading day of the year). Adding 2021-01-04 (the first trading day of the new year) to the CSV fixed that issue.

I'm not sure where exactly the bug is located, but it has to do with creating a calendar with an end date x, where the last trading day before x is a special close (or open?) day. Maybe worth checking.

This doesn't show up e.g. for XETR since the corresponding CSV doesn't stops a few days before Christmas, so it doesn't end with the last trading day of the year which is also a half day.

[gerrymanoim]

Thanks - good catch here. All the code changes LGTM.

I suspect that there's some issue with searching through the array of holidays. Do you mind filing an issue on that and I'll take a look?

[jenskeiner]

Thanks - good catch here. All the code changes LGTM.

I suspect that there's some issue with searching through the array of holidays. Do you mind filing an issue on that and I'll take a look?

sure