XTAI: Add missing holidays

[w3stling]

Fixes #511
Add the missing holidays — Teachers’ Day (September 28), Taiwan Restoration Day (October 25), and Constitution Day (December 25) — for TWSE.

Source:
https://www.twse.com.tw/en/trading/holiday.html

Workflow to modify an existing Exchange Calendar

• Modify calendar class as required.
• Modify the test resources file (e.g tests/resources/{Exchange MIC}.csv), either manually or by executing python etc/make_exchange_calendar_test_csv.py {Exchange MIC}.
• Check if any of the fixtures in tests/test_{Exchange MIC}_calendar.py need updating to reflect your changes.
• Add references to any new/modified holidays in exchange_calendars/exchange_calendar_{Exchange MIC}.py.