Development shell for pyproject.toml with a dynamic version

I would like to patch a calendar tool GitHub - pimutils/khal: 📆 CLI calendar application
It uses a pyproject.toml which I associated to poetry but seems like this one uses setuptools as a builder ?! Anyway I tried

# from a clone of the project
$ nix develop nixpkgs#khal
$ python3 bin/ikhal
Failed to find (autogenerated) This might be due to using GitHub's tarballs or svn access. Either clone from GitHub via git or get a tarball from PyPI.

This apparentrly comes from the dynamic = ["version"] in the pyproject.toml. I tried to comment it and added a static version to no avail.
I also tried to write manually the expected file (I deduced from this part of the pyproject.toml:

version_file = "khal/"

with little chance

  echo "__version__ = '0.1'" >> khal/ 
khal on  master via 🐍 v3.11.8 via ❄️  IMPURE (khal-0.11.3-env) 
➜ python3 khal/
Traceback (most recent call last):
  File "/home/teto/khal/khal/", line 33, in <module>
    from . import __version__, controllers, khalendar
ImportError: attempted relative import with no known parent package

Happy to take any advice on how to test the changes I make to khal via an editable install