In #6620 setup.py was largely simplified but still some bits remain in old setup.py files. Now with lates pip and setuptools installing extensions as editable for development, there are following warnings:

DEPRECATION: Legacy editable install of <some_extension> from file:///srv/app/ckanext/<some_extension> (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at https://github.com/pypa/pip/issues/11457

Should we try to solve the remaining issues and start transitioning to pyproject.toml configurations in extensions and core or do we want to maintain setup.py/setup.cfg configurations ?