Drop runtime pkg_resources dependency for setuptools >= 82 compatibility

[pmrv]

Pull Request Template

Description of Changes

setuptools 82 removed the pkg_resources module entirely (it was deprecated in 81), which broke this package at runtime: bin/pacemaker and src/pyace/multispecies_basisextension.py failed at import time with ModuleNotFoundError: No module named 'pkg_resources' when installed alongside setuptools > 81.

• Replaced pkg_resources.resource_filename(...) with importlib.resources.files(...) in bin/pacemaker and src/pyace/multispecies_basisextension.py, with a pkg_resources fallback for Python 3.8 (the oldest supported version, where importlib.resources.files is unavailable). This removes the implicit runtime dependency on setuptools altogether.
• Removed the unused from distutils.version import LooseVersion import in setup.py (distutils was removed from the stdlib in Python 3.12).

Verified in a clean venv with setuptools 83.0.0 (no pkg_resources module present): the importlib.resources lookup resolves pyace.data resource paths correctly, and setup.py egg_info still succeeds.

Checklist

• Code is well-documented.
• All tests have been run and passed.
• Relevant documentation has been updated if necessary.

License Agreement

By submitting this pull request, I agree that:

• The code submitted in this pull request will be distributed under the Academic Software License (free for academic non-commercial use, not free for commercial use), see LICENSE.md for more details.
• The copyright for the code, including the submitted code, remains with Ruhr University Bochum. Ruhr University Bochum retains the right to transfer or modify the copyright.

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[pmrv]

lgtm merge