dh: In v9, do not enable any python support commands.

dh: In v9, do not enable any python support commands.

dh_pysupport has started emitting a deprecation warning, which is
very annoying since it clutters every build that uses dh -- even builds
where it doesn't do anything. Since there is not just a dh_python2, but
also a dh_python3 waiting in the wings, this is clearly too volatile
a situation for dh to try to support further.

I considered making dh_python detect and run the right dh_python[23] helper
-- a python helper helper as it were -- but 70-odd packages still use that
command.