Fix failing CI test

The CI complained that our import of the `twiss` helper function is unused.
But we need it to be available to use it in our examples.

src/python/impactx/__init__.py

@@ -18,7 +18,7 @@
 
 # import core bindings to C++
 from . import impactx_pybind as cxx
-from .distribution_input_helpers import twiss
+from .distribution_input_helpers import twiss  # noqa
 from .extensions.ImpactXParIter import register_ImpactXParIter_extension
 from .extensions.ImpactXParticleContainer import (
     register_ImpactXParticleContainer_extension,