I know very little about the details of OpenStack, but in case a somewhat
parallel example is useful, that's approximately what Django will do.
Bullseye will be Django 2.0, which is Python 3 only. Buster is the pivot
release where the third party elements of the Django ecosystem almost all
support Python 3, so transition is possible to make ready for an all Python 3
future. (AIUI anyway, I'm not a Django maintainer)