Add Provides: dh-sequence-python3 and dh-sequence-pypy

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

Other Info

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.)