Add stub for pathlib2 on Python 2.7 (#2351)

pathlib2 is the Python 2.7 backport of the pathlib module from Python 3.
Hence we use the same stub file for both.

The maintainer of pathlib2 granted permission for stubs to be added in
mcmtroffaes/pathlib2#44.

stdlib/3/pathlib.pyi

@@ -1,5 +1,3 @@
-# Stubs for pathlib (Python 3.4)
-
 from typing import Any, Generator, IO, Optional, Sequence, Tuple, Type, TypeVar, Union, List
 from types import TracebackType
 import os