My python builds started failing 5 days ago. After doing some troubleshooting looks like pip was upgraded to pip 19.0 and that is causing my builds to break. pip v19.0 appears to have a bug that doesn’t install packages such as flask & pyinstaller.
I have found a work around which gets the builds back up but the bug in pip should really be fixed moving forward.
Here is the work around:
Downgrade to pip v18.0 before installing any dependencies
pip install --user pip==18.0.0
After downgrading the pip client to v18 your previous builds should all work as before.