pkg_resources is deprecated, pesummary needs to stop using it

pkg_resources has been deprecated, see https://setuptools.pypa.io/en/latest/pkg_resources.html, but pesummary is using it:

$ PYTHONWARNINGS="error::DeprecationWarning" python3 -c "import pesummary.conf.configuration"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/duncan/git/pesummary/pesummary/conf/__init__.py", line 1, in <module>
    from .configuration import *
  File "/home/duncan/git/pesummary/pesummary/conf/configuration.py", line 2, in <module>
    import pkg_resources
  File "/home/duncan/opt/mambaforge/envs/py311/lib/python3.11/site-packages/pkg_resources/__init__.py", line 118, in <module>
    warnings.warn(
DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html

This project needs to be updated to ... not use it.