Added public holidays before 1999 in XSHG.

[qiuwei]

Workflow to add a new Exchange Calendar

It's recommended that whilst working through the process reference be made to existing calendar and test classes.

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 fixtrues 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.

[gerrymanoim]

This is great - Thanks! You can ignore the CI error, that will be fixed via #146.