pip no longer working after update error 'module' object is not callable

All credit for this goes to user han_solo, who left the answer as a comment, instead of as an answer:

Instead of pip install stdlib_list --user

use python -m pip install stdlib_list --user

Replace stdlib_list with matplotlib or whatever the name is of the package you want.

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.