fix sys.exc_info return type (#246)

stdlib/2.7/sys.pyi

@@ -108,7 +108,11 @@ def __displayhook__(value: int) -> None: ...
 def __excepthook__(type_: type, value: BaseException, traceback: TracebackType) -> None: ...
 def exc_clear() -> None:
     raise DeprecationWarning()
-def exc_info() -> Tuple[type, BaseException, TracebackType]: ...
+# TODO should be a union of tuple, see mypy#1178
+def exc_info() -> Tuple[Optional[type],
+                        Optional[BaseException],
+                        Optional[TracebackType]]: ...
+
 # sys.exit() accepts an optional argument of anything printable
 def exit(arg: Any = ...) -> None:
     raise SystemExit()

stdlib/3/sys.pyi

@@ -117,7 +117,10 @@ def _current_frames() -> Dict[int, Any]: ...
 def displayhook(value: Optional[int]) -> None: ...
 def excepthook(type_: type, value: BaseException,
                traceback: TracebackType) -> None: ...
-def exc_info() -> Tuple[type, BaseException, TracebackType]: ...
+# TODO should be a union of tuple, see mypy#1178
+def exc_info() -> Tuple[Optional[type],
+                        Optional[BaseException],
+                        Optional[TracebackType]]: ...
 # sys.exit() accepts an optional argument of anything printable
 def exit(arg: Any = ...) -> None:
     raise SystemExit()