fix some abstract classes in Python 2 (#2240)

Part of #1476.

stdlib/2/builtins.pyi

@@ -1060,6 +1060,7 @@ class file(BinaryIO):
     @overload
     def __init__(self, file: int, mode: str = ..., buffering: int = ...) -> None: ...
     def __iter__(self) -> Iterator[str]: ...
+    def next(self) -> str: ...
     def read(self, n: int = ...) -> str: ...
     def __enter__(self) -> BinaryIO: ...
     def __exit__(self, t: Optional[type] = ..., exc: Optional[BaseException] = ..., tb: Optional[Any] = ...) -> bool: ...