Return a date corresponding to a date_string given in any valid ISO 8601 format, with the following exceptions:
classmethod date.fromisoformat(date_string)
>>> from datetime import date
>>> date.fromisoformat('2019-12-04')
datetime.date(2019, 12, 4)
>>> date.fromisoformat('20191204')
datetime.date(2019, 12, 4)
>>> date.fromisoformat('2021-W01-1')
datetime.date(2021, 1, 4)