Apply stub fixes from o11c.

These are extracted from https://github.com/JukkaL/mypy/pull/721

builtins/3/sys.pyi

@@ -4,7 +4,7 @@
 # based on http://docs.python.org/3.2/library/sys.html
 
 from typing import (
-    List, Sequence, Any, Dict, Tuple, TextIO, overload, Optional
+    List, Sequence, Any, Dict, Tuple, TextIO, overload, Optional, Union
 )
 from types import TracebackType
 
@@ -115,7 +115,7 @@ def displayhook(value: Optional[int]) -> None: ...
 def excepthook(type_: type, value: BaseException,
                traceback: TracebackType) -> None: ...
 def exc_info() -> Tuple[type, BaseException, TracebackType]: ...
-def exit(arg: int = None) -> None: ...
+def exit(arg: Union[int, str] = ...) -> None: ...
 def getcheckinterval() -> int: ...  # deprecated
 def getdefaultencoding() -> str: ...
 def getdlopenflags() -> int: ...  # Unix only