Now packages can Build-Depend on dh-sequence-python3 instead of dh-python and then drop --with python3 from debian/rules
Requires debhelper 12
https://manpages.debian.org/unstable/dh
I think it would be useful for this change to make it into Buster so that this feature can be more easily used in Bullseye. (Easier for backports and for people doing packaging stuff on Buster.)