Copy __builtin__.pyi to builtins.pyi, to fix breakage caused by #2128 (#2215)

Author: gvanrossum

stdlib/2/builtins.pyi

@@ -29,7 +29,7 @@ class object:
     __doc__ = ...  # type: Optional[str]
     __class__ = ...  # type: type
     __dict__ = ...  # type: Dict[str, Any]
-    __slots__ = ...  # type: Optional[Union[str, unicode, Iterable[Union[str, unicode]]]]
+    __slots__ = ...  # type: Union[str, unicode, Iterable[Union[str, unicode]]]
     __module__ = ...  # type: str
 
     def __init__(self) -> None: ...