change the way 'os' imports 'os.path' (#601)

* use 'from . import path' instead, in both PY2 and PY3
* update blacklist to include os/__init__.pyi

stdlib/3/os/__init__.pyi

@@ -9,7 +9,7 @@ from typing import (
 )
 import sys
 from builtins import OSError as error
-import os.path as path
+from . import path
 
 # ----- os variables -----