I think the problem is in commit [6718d1e]. python3-tz was dropped from Depends, but the proper fix would have been to use python3-pytz instead, as some packages which build-depend on such binary package rely on such dependency to be there.
If nobody objects, I will readd the missing Depends via team upload. Not that we will ask for this package to be unblocked but at least we will start un-breaking unstable. Thanks.
