update deprecated use of pkg_resources

[orbeckst]

We use pkg_resources:

DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    from pkg_resources import resource_string

Use importlib instead, along the line of

import importlib.resources as importlib_resources

__all__ = ["DX", "CCP4", "gOpenMol"]

DX = importlib_resources.files(__name__) / 'test.dx'

(from https://github.com/MDAnalysis/GridDataFormats/blob/master/gridData/tests/datafiles/__init__.py where this was recently fixed)

[IAlibay]

I'm surprised, I thought that would have failed with Python 3.12 O.O!

[orbeckst]

It just versioneer... need to update.

[orbeckst]

Well, I am wrong: MDAnalysisData/base.py:from pkg_resources import resource_string