Replace pkg_resources with Importlib_resources

[mdpiper]

The pkg_resources package has been deprecated and removed from setuptools. It's recommended to use importlib_resources instead.

[mdpiper]

It looks like pkg_resources is only used once.

$ grep -r pkg_resources pymt
pymt/model_collection.py:3:import pkg_resources
pymt/model_collection.py:21:        for entry_point in pkg_resources.iter_entry_points(group="pymt.plugins"):