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.

[afrendeiro]

In xarray this dependency was removed a while ago when python 3.7 support ended and it had very slow import times:
pydata/xarray#5676

[quentinblampey]

Any news about this @jhamman? Some of my packages and CI are crashing because of this, it would be very nice to make a new version without pkg_resources

[quentinblampey]

I made a PR here to fix this