I ran into the same problem and what I noticed is that the pip in the default distro is old 1.5.6 the current released version being 7.1.0. Since is hard to track down the issue, looking there and there for the "needle in a hay stack" I removed it and installed it manually as suggested in the following :
This comment : https://github.com/pypa/pip/issues/1093#issuecomment-103127883
referenced in this stack: https://stackoverflow.com/questions/19460232/pip-list-crashes-with-an-assertionerror
Remove pip:
sudo apt-get remove --auto-remove python-pip
Download the install script :
wget https://bootstrap.pypa.io/get-pip.py
Install it back:
sudo python get-pip.py
Link commands to bin :
sudo ln -s [PIP_HOME]/bin/pip /usr/local/bin/pip
sudo ln -s [PIP_HOME]/bin/wheel /usr/local/bin/wheel
sudo ln -s [PIP_HOME]/bin/easy_install /usr/local/bin/easy_install
Where [PIP_HOME] is the directory of the pip package.
now when I run pip -V
puts out :
pip 7.1.0 from /home/tiberiu/.local/lib/python2.7/site-packages (python 2.7)
Back to my root problem the command pip install -r requirements.txt -t lib
success with the newest version.