Ubuntu comes with a version of PIP from precambrian and that’s how you have to upgrade it if you do not want to spend hours and hours debugging pip related issues.
apt-get remove python-pip python3-pip
wget https://bootstrap.pypa.io/get-pip.py
python get-pip.py
python3 get-pip.py
As you observed I included information for both Python 2.x and 3.x