Investigate unit test failures with python 3.6, 3.7, 3.12

- 3.6: Not available on ubuntu-22.04
- 3.7: Results in failures in `test_cli.py`
- 3.12: pkg_resources not supported
- ```
pydicom/data/data_manager.py:112: in get_external_sources
from pkg_resources import iter_entry_points
E ModuleNotFoundError: No module named 'pkg_resources'
- Probably due to loading incorrect version of `pydicom`. The latest version does not use `pkg_resources`, changed in [Commit f5eeee](https://github.com/pydicom/pydicom/commit/f5eeee05a99cd296350dd87fa6267cc5fa8a81fb#diff-154162c110cdd1950877a51216324a26aa7f68980239cf96c1ffd23a49658428)

Reproduce the reported Python-version results, then inspect test_cli.py and pydicom/data/data_manager.py, using commit f5eeee as the reference for the pkg_resources change. Done means the failures for the supported Python versions are explained and resolved, with the unavailable Python 3.6 case accounted for.

Written by the indexing model from the issue text.