2018-03-19 pip 9.0.2 and issue with pip._vendor.urllib3.contrib

pip was recently updated and the new version produces a weird error:

KeyError: 'pip._vendor.urllib3.contrib'

The error happens pip/_vendor/requests/packages.py. It seems related to issue 5081. The best is to keep pip 9.0.1.:

python -m pip install pip!=9.0.2