Migrate from setup.py to pyproject.toml + setup.cfg (PEP 660)

[pabloitu]

Installing the package in editable mode using setup.py develop or pip install -e . currently triggers a deprecation warning. This is due to the reliance on the legacy build system. Future versions of pip (v25.1+) will remove support for legacy editable installs, breaking this workflow unless we adopt the modern PEP 660-compliant setup using pyproject.toml.

DEPRECATION: Legacy editable install of pycsep[all]==0.6.3 from file:///... (setup.py develop) is deprecated. pip 25.1 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

Recommend to create pyproject.toml with the minimal specs, a setup.cfg for metadata information and remove setup.py or keep it minimal.