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