I tried with every answer avobe, but couldn't make it.
Did this and worked
sudo apt-get purge python-virtualenv
sudo pip install pip -U
After that I just installed virtualenv with pip
sudo pip install virtualenv
I built the virtualenv that I was working on and
the package was installed easily.
Get into the virtualenv by using source /bin/activate
and try to install your package, for example:
pip install terminado
It worked for me, although I was using python2.7 not python3