Add date.fromisoformat() (#2393)

stdlib/2and3/datetime.pyi

@@ -43,6 +43,9 @@ class date:
     def today(cls) -> date: ...
     @classmethod
     def fromordinal(cls, n: int) -> date: ...
+    if sys.version_info >= (3, 7):
+        @classmethod
+        def fromisoformat(cls, date_string: str) -> date: ...
 
     @property
     def year(self) -> int: ...