Add NoReturn (#811)

* Add NoReturn

stdlib/3/sys.pyi

@@ -8,6 +8,7 @@ from typing import (
     TypeVar, Callable, Type,
 )
 from types import TracebackType
+from mypy_extensions import NoReturn
 
 _T = TypeVar('_T')
 
@@ -120,7 +121,7 @@ def exc_info() -> Tuple[Optional[Type[BaseException]],
                         Optional[BaseException],
                         Optional[TracebackType]]: ...
 # sys.exit() accepts an optional argument of anything printable
-def exit(arg: Any = ...) -> None:
+def exit(arg: Any = ...) -> NoReturn:
     raise SystemExit()
 def getcheckinterval() -> int: ...  # deprecated
 def getdefaultencoding() -> str: ...