XSHG: Add 2023-10-06 as a holiday

[philiptromans]

This PR adds 2023-10-06 as a holiday in XSHG. It's the last working day of the Mid-Autumn Festival and China’s National Day holiday. SSE source: http://english.sse.com.cn/start/trading/schedule/

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.

[maread99]

Thanks @philiptromans. I'll merge after # 329.