Update docstring for Holiday
Renamed weekend_hint to observance_hint

--HG--
extra : amend_source : 8ecc724430c4ea14fdc35f99b014d23c2fff75d1
Moved the observance calculation to the calendar.
Extracted variable for should_shift
Re-implemented the 'observance_hint' as an 'observance_shift', for which the default is now defined in the Calendar class.
The observance shift test now honors Calendar.get_weekend_days