Transition Python packages to use dh_python helpers

We have had python-central and python-support for a long time. Now we have dh_python2 (for Python 2.x) and dh_python3 (for Python 3.x) which are the new goodness for Python packages. All python-central and python-support packages should be converted to dh_python during Oneinic cycle, with coordination through Debian. By coordination, I mean submitting bug reports and patches to the Debian maintainers, but not necessarily waiting on them to do the conversion. We'll make a case-by-case decision about that based on the need to sync and maintain our own set of patches.

python-central -> dh_python2 should be very smooth since the python-central maintainer has already declared it obsolete.