After a pip update, pip has stopped working completely.
Z:\\>pip install matplotlib Traceback (most recent call last): File \"c:\\program files\\python3
I've had the same issue as you, and I solved it uninstalling pip and installing again.
To uninstall: python -m pip uninstall pip
To install, follow the instructions: https://www.liquidweb.com/kb/install-pip-windows/
After that you will going to have a older but functional 19.0.3 version.