Update pyproject.toml - use poetry-core

[Spitfire1900]

 × python setup.py develop did not run successfully.
    │ exit code: 1
    ╰─> [1 lines of output]
        ERROR: Can not execute `setup.py` since setuptools is not available in the build environment.
        [end of output]

Migrate to poetry-core for build-system, which does not produce the above error on an editable install pip install -e.