rules: Pass the private module directory to dh_python3

We only build for the default python version. Previously dh_python3
wasn't finding the directory containing the private modules, so it
didn't know this. This meant that no versioned python3 dependencies were
generated so we could, for example, be migrated without the
corresponding version of python3 migrating. That results in programs
which don't run.

Once the directory is passed, dh_python3 generates dependencies on the
right python versions. For future transitions gobject-introspection will
need to be binNMUed.

Bug-Debian: #950267