calendar: add firstweekday property (#5325)

Co-authored-by: hauntsaninja <>

stdlib/calendar.pyi

@@ -19,6 +19,7 @@ def weekday(year: int, month: int, day: int) -> int: ...
 def monthrange(year: int, month: int) -> Tuple[int, int]: ...
 
 class Calendar:
+    firstweekday: int
     def __init__(self, firstweekday: int = ...) -> None: ...
     def getfirstweekday(self) -> int: ...
     def setfirstweekday(self, firstweekday: int) -> None: ...