python/pysixtracklib: fixes an import issue with a submodule in pysixtrack_helper.py
python/pysixtracklib: fixes an import issue with a submodule in pysixtrack_helper.py

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib