Inkscape need a certain number a Python modules at runtime.
There is a top level requirement.txt to that contain all the modules
that can be installed using the standard mechanism.
The corresponding manifest fragment is python3-requirements.json. It
is generated using flatpak-builder-tools.
./pip/flatpak-pip-generator -r PATH_TO/org.inkscape.Inkscape/requirements.txt
Unfortunately lxml is not in the Platform but is in the Sdk. So it needs
to be force installed.