fix: dev env setup script
Without these changes, ./scripts/dev_env.sh
fails with:
ERROR: Project file:///home/nicoco/src/perso/xmpp/gajim has a 'pyproject.toml' and its build backend is missing the 'build_editable' hook. Since it does not have a 'setup.py' nor a 'setup.cfg', it cannot be installed in editable mode. Consider using a build backend that supports PEP 660.
Maybe we need to do something more subtle somewhere, but this seems to work.