os.path.exists (Py 3) accepts a file descriptor

Closes python#1653

Author: srittau

stdlib/3/os/path.pyi

@@ -100,7 +100,7 @@ elif sys.version_info >= (3, 5):
 # So, fall back to Any
 def commonprefix(list: Sequence[_PathType]) -> Any: ...
 
-def exists(path: _PathType) -> bool: ...
+def exists(path: Union[_PathType, int]) -> bool: ...
 def lexists(path: _PathType) -> bool: ...
 
 # These return float if os.stat_float_times() == True,