Remove pkg_resources runtime dependency

[Mirochill]

Summary

• replace pkg_resources.resource_filename() with importlib.resources when
  reading the packaged phospho-SLiM data file
• remove the runtime setuptools dependency that was only needed for
  pkg_resources, while retaining setuptools as the build backend

Rationale

mavisp/modules.py currently imports pkg_resources while loading packaged
data. Recent setuptools installations no longer provide that module, which
prevents the MAVISp package from being used in those environments. The package
already requires Python 3.12 or newer, so the standard-library resource API can
provide the same packaged-file access without a setuptools runtime dependency.

Fixes #392

Validation

• git diff --check HEAD~1..HEAD
• static search for remaining pkg_resources references in mavisp and
  pyproject.toml
• Not run locally