Some time ago, I posted a bpo-36004, proposing the addition of a `date.fromisocalendar` alternate constructor, and the corresponding PR #11888 implementing it.

This function is the inverse of `date.isocalendar`. I think that, whenever possible, if we have a function that serializes a date or datetime, we should have a conjugate function that constructs a date/datetime from the output; the big one of these was `datetime.fromisoformat`, which was added in Python 3.7, but I have noticed that `isocalendar` also does not currently have an inverting function, and I'd like to add that for Python 3.8.

It has been finished for a while, but I would like some additional review of both the idea and implementation, so I'd like to bring it to the attention of the list.

Best,
Paul