Update deprecated `pkg_resources` usage

[quentinblampey]

Hello,

I get this warning during the package import. Is it planned to update it?

/XXX/.venv/lib/python3.13/site-packages/xarray_schema/__init__.py:1: UserWarning: pkg_resources is deprecated as an API.
See https://setuptools.pypa.io/en/latest/pkg_resources.html.
The pkg_resources package is slated for removal as early as 2025-11-30.
Refrain from using this package or pin to Setuptools<81.