normalize pyproject, PEP-621 compliant

[mgaitan]

Remove setup.py and keep only pyproject.toml in a PEP-621 compliance format, using flit to build . The description adn version are declared explicitly in __init__.py module and loaded dynamically for the package.

This fixes "No module named 'pkg_resources'" when setuptools is not available.

$  uv pip install sphinxcontrib-kroki
Resolved 23 packages in 376ms
Installed 1 package in 6ms
 + sphinxcontrib-kroki==1.3.0

  docs_diagrams [$!++(1)?]   (.docs-venv)
Shiphero-API ❯ make docs quick
make[1]: se entra en el directorio '/home/tin/lab/Shiphero-API/docs'
Ejecutando Sphinx v7.1.2

Extension error:
No puede importar la extensión sphinxcontrib.kroki (exception: No module named 'pkg_resources')
make[1]: *** [Makefile:30: quick] Error 2
make[1]: se sale del directorio '/home/tin/lab/Shiphero-API/docs'
make: *** [Makefile:161: docs] Error 2

[mgaitan]

added this because wasn't declared as dep

[danieleades]

this isn't required, black will infer it from the requires-python value

see https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#t-target-version