Guido van Rossum wrote
My suggestion is that the Python micro-release number be included in the path to system-packages. IOW, system-packages must exactly match the Python version number, not just the maj.min number.
Excellent point. Or a micro-release should clear out the system-packages directory.
That's just a bit too sudden and nasty. If this is the way to go, make a subdirectory of system-packages/site-upgrades and move the old files there (and warn the installer that they've been moved)