pip on requirement.txt gives "This backport is for Python 2.7 only." for functool32

[stevenluda]

When
pip install -r requirements.txt
,
the following error occurred:
Collecting appnope==0.1.0 (from -r requirements.txt (line 1))
Using cached appnope-0.1.0-py2.py3-none-any.whl
Collecting backports.ssl-match-hostname==3.4.0.2 (from -r requirements.txt (line 2))
Using cached backports.ssl_match_hostname-3.4.0.2.tar.gz
Collecting certifi==2015.9.6.2 (from -r requirements.txt (line 3))
Using cached certifi-2015.9.6.2-py2.py3-none-any.whl
Collecting decorator==4.0.2 (from -r requirements.txt (line 4))
Using cached decorator-4.0.2-py2.py3-none-any.whl
Collecting funcsigs==0.4 (from -r requirements.txt (line 5))
Using cached funcsigs-0.4-py2.py3-none-any.whl
Collecting functools32==3.2.3.post2 (from -r requirements.txt (line 6))
Using cached functools32-3.2.3-2.zip
Complete output from command python setup.py egg_info:
This backport is for Python 2.7 only.

----------------------------------------

Command "python setup.py egg_info" failed with error code 1 in C:\Users\lud\AppData\Local\Temp\pip-build-oboinpbr\functools32\

My environment is
Python: 3.6.4
Pip: 9.0.1

Any suggestion?