Correct return type of sum() builtin (#1582)

`sum([])` always returns the integer 0.

Author: teepark

stdlib/2/__builtin__.pyi

@@ -789,7 +789,10 @@ def sorted(iterable: Iterable[_T], *,
            cmp: Callable[[_T, _T], int] = ...,
            key: Callable[[_T], Any] = ...,
            reverse: bool = ...) -> List[_T]: ...
-def sum(iterable: Iterable[_T], start: _T = ...) -> _T: ...
+@overload
+def sum(iterable: Iterable[_T]) -> Union[_T, int]: ...
+@overload
+def sum(iterable: Iterable[_T], start: _S) -> Union[_T, _S]: ...
 def unichr(i: int) -> unicode: ...
 def vars(object: Any = ...) -> Dict[str, Any]: ...
 @overload

stdlib/3/builtins.pyi

@@ -861,7 +861,10 @@ def setattr(object: Any, name: str, value: Any) -> None: ...
 def sorted(iterable: Iterable[_T], *,
            key: Optional[Callable[[_T], Any]] = None,
            reverse: bool = False) -> List[_T]: ...
-def sum(iterable: Iterable[_T], start: _T = ...) -> _T: ...
+@overload
+def sum(iterable: Iterable[_T]) -> Union[_T, int]: ...
+@overload
+def sum(iterable: Iterable[_T], start: _S) -> Union[_T, _S]: ...
 def vars(object: Any = ...) -> Dict[str, Any]: ...
 @overload
 def zip(iter1: Iterable[_T1]) -> Iterator[Tuple[_T1]]: ...