devel/py-dbus: Decouple from pydbus-common and enable testing

These ports were split more than 10 years ago in a1fb7d3e4ebf09080fe48ea40b2cb801a56cf710
to support both Python 2 and Python 3. Later the py3-dbus variant got removed,
so there is no point in having devel/pydbus-common now.

git archeology by: jhale