COMPAT: Implement Timestamp.fromisocalendar
https://docs.python.org/3.8/whatsnew/3.8.html#datetime / https://docs.python.org/3.8/library/datetime.html#datetime.datetime.fromisocalendar

> Added new alternate constructors datetime.date.fromisocalendar() and datetime.datetime.fromisocalendar(), which construct date and datetime objects respectively from ISO year, week number and weekday; these are the inverse of each class’s isocalendar method. (Contributed by Paul Ganssle in bpo-36004.)
