pip install -e src/rogtk/

import rogtk.rogtk as rogtk
rogtk.sum_as_string(1, 2)

The issue was that even after running maturin develop (or build with or
without --release) there were some .so files for the python package that were
not overwritten. That's why starting from scratch worked (once) since those
original files were gone. After I delete it (together with the __pycache__ dir)
the python side is up to date.