Support for Python 3.13

[jtniehof]

This PR makes updates for Python 3.13 support

• Changes the CI and build/test scripts to work against 3.13. There's some more general cleanup of the build/test scripts that was really enabled by #749 but it just got annoying not to do it here.
• Fix some invalid docstring formatting that only showed up once testing against the newer sphinx on 3.13.
• Fixes tests for the deprecation decorator test due to python/cpython#81283
• Fixes a numpy 1.25 deprecation in test_coordinates. This wasn't raising an error since it was associated with numpy.testing, not spacepy code.
• Works around the deprecation of pkg_resources in irbempy. irbempy uses importlib.resources on 3.9+, but 3.8 is still getting updates to setuptools which deprecated pkg_resources. The workaround tries to use the older version of importlib.resources where it can, only punting to pkg_resources where it fails (on zip / egg import). We picked up an undeclared runtime dependency on pkg_resources (setuptools) in 5a920b9af7bc0d0142f51bee8ad9a1ae523039e9.

PR Checklist

• [X] Pull request has descriptive title
• [X] Pull request gives overview of changes
• [X] (N/A) New code has inline comments where necessary
• [X] (N/A) Any new modules, functions or classes have docstrings consistent with SpacePy style
• [X] (N/A?) Added an entry to release notes if fixing a significant bug or providing a new feature
• [X] New features and bug fixes should have unit tests
• [X] Relevant issues are linked to (e.g. See issue # or Closes #)

No release note at this point since there were no changes to the spacepy code required to work with 3.13.