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/2.7/os/__init__.pyi

@@ -4,7 +4,7 @@ from typing import (
     List, Tuple, Union, Sequence, Mapping, IO, Any, Optional, AnyStr, Iterator,
     MutableMapping, NamedTuple, overload
 )
-from os import path
+from . import path
 
 error = OSError
 name = ... # type: str