Add itertools.repeat.__length_hint__ method (#7212)

Author: AlexWaygood

stdlib/itertools.pyi

@@ -49,6 +49,7 @@ class repeat(Iterator[_T], Generic[_T]):
     def __init__(self, object: _T, times: int) -> None: ...
     def __next__(self) -> _T: ...
     def __iter__(self: Self) -> Self: ...
+    def __length_hint__(self) -> int: ...
 
 class accumulate(Iterator[_T], Generic[_T]):
     if sys.version_info >= (3, 8):