datetime: add timedelta.__bool__ (#5555)

stdlib/datetime.pyi

@@ -179,6 +179,7 @@ class timedelta(SupportsAbs[timedelta]):
     def __lt__(self, other: timedelta) -> bool: ...
     def __ge__(self, other: timedelta) -> bool: ...
     def __gt__(self, other: timedelta) -> bool: ...
+    def __bool__(self) -> bool: ...
     def __hash__(self) -> int: ...
 
 class datetime(date):