I suppose you could also clean out cached packages with pacman -Scc
and then delete the package database files in /var/lib/pacman/sync
so that you know for a fact that you are getting a fresh sync and it not depending on anything cached.
but the package is certainly there, are you able to directly download it from https://arch.mirror.constant.com/extra/os/x86_64/python-polib-1.2.0-2-any.pkg.tar.zst from a browser so we can rule out any weird dns fuckiness?
Entertain me, what's the output of
pacman -Qi python| grep Architecture
?