Remove use of pkg_resources package

[paulromano]

Description

I noticed that running tests now gives the following warning:

conftest.py:3
  /home/romano/openmc/tests/regression_tests/conftest.py:3: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    from pkg_resources import parse_version

This PR addresses this by replacing use of the pkg_resources package with importlib.resources and packaging, which are the recommended replacements.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

Looks good! Thanks @paulromano.

Placing the link in the PR description here for future reference if needed:
https://setuptools.pypa.io/en/latest/pkg_resources.html