Replace pkg_resources for example data paths

[Mirochill]

Summary

• replace pkg_resources.resource_filename() in read_example() with importlib.resources
• remove the remaining pkg_resources imports from the bundled example data package and test resource helper
• add a changelog entry for the example-data resource lookup change

Reported downstream in iosonobert/pIMOS#32.

Validation

• git diff --check
• Not run locally