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.)

debian/control

@@ -20,6 +20,7 @@ Multi-Arch: foreign
 Depends: ${misc:Depends}, ${python3:Depends},
 # depend on distutils to avoid adding it to Build-Depends in other packages
  python3-distutils | python3 (<< 3.6.5~rc1-1)
+Provides: dh-sequence-python3, dh-sequence-pypy
 Suggests: libdpkg-perl, dpkg-dev
 Breaks:
 # due to /usr/bin/dh_python3 and debhelper files