tests/test_cboe_calendars.py proposed enhancement

[dougransom]

https://github.com/rsheftel/pandas_market_calendars/blob/master/tests/test_cboe_calendars.py

Could potentially download a series that should trade every day, like vix futures, and make sure the holidays match with actual trading days each month. If they don't, could be an unusual market event OR a problem with the code.

[rsheftel]

That is an excellent idea. If you do that in a Pull Request I will review.