Allow os.readlink to accept path-like objects (#1441)

Author: rhysparry

stdlib/3/os/__init__.pyi

@@ -323,7 +323,10 @@ def mkdir(path: _PathType, mode: int = ...) -> None: ...
 def makedirs(path: _PathType, mode: int = ...,
              exist_ok: bool = ...) -> None: ...
 def pathconf(path: _PathType, name: Union[str, int]) -> int: ...  # Unix only
-def readlink(path: AnyStr) -> AnyStr: ...
+if sys.version_info >= (3, 6):
+    def readlink(path: Union[AnyStr, PathLike[AnyStr]]) -> AnyStr: ...
+else:
+    def readlink(path: AnyStr) -> AnyStr: ...
 def remove(path: _PathType) -> None: ...
 def removedirs(path: _PathType) -> None: ...
 def rename(src: _PathType, dst: _PathType) -> None: ...