Make islice start and stop parameters optional (#2031)

From the documentation [1]: "If start is None, then iteration starts at zero."

[1] https://docs.python.org/3/library/itertools.html#itertools.islice.

PR #1603 made this change for Python 2.

Author: svend

stdlib/2/itertools.pyi

@@ -38,7 +38,7 @@ def groupby(iterable: Iterable[_T],
             key: Callable[[_T], _S]) -> Iterator[Tuple[_S, Iterator[_T]]]: ...
 
 @overload
-def islice(iterable: Iterable[_T], stop: int) -> Iterator[_T]: ...
+def islice(iterable: Iterable[_T], stop: Optional[int]) -> Iterator[_T]: ...
 @overload
 def islice(iterable: Iterable[_T], start: Optional[int], stop: Optional[int],
            step: int = ...) -> Iterator[_T]: ...

stdlib/3/itertools.pyi

@@ -40,9 +40,9 @@ def groupby(iterable: Iterable[_T],
             key: Callable[[_T], _S]) -> Iterator[Tuple[_S, Iterator[_T]]]: ...
 
 @overload
-def islice(iterable: Iterable[_T], stop: int) -> Iterator[_T]: ...
+def islice(iterable: Iterable[_T], stop: Optional[int]) -> Iterator[_T]: ...
 @overload
-def islice(iterable: Iterable[_T], start: int, stop: Optional[int],
+def islice(iterable: Iterable[_T], start: Optional[int], stop: Optional[int],
            step: int = ...) -> Iterator[_T]: ...
 
 def starmap(func: Callable[..., _S], iterable: Iterable[Iterable[Any]]) -> Iterator[_S]: ...