Drop runtime pkg_resources dependency for setuptools >= 82 compatibility

[pmrv]

setuptools 82 removed the pkg_resources module entirely, which broke 'pacemaker' and pyace.multispecies_basisextension at import time (ModuleNotFoundError: No module named 'pkg_resources').

Replace pkg_resources.resource_filename with importlib.resources.files, keeping a pkg_resources fallback for Python 3.8 where importlib.resources.files is unavailable. Also drop the unused distutils.version.LooseVersion import from setup.py (distutils was removed from the stdlib in Python 3.12).

Description of Changes

Checklist

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

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• 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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