Move test requirements to dependency-groups

Xref https://peps.python.org/pep-0735 and https://packaging.python.org/en/latest/specifications/dependency-groups/#dependency-groups. Added missing netcdf4 and pooch dependencies, and updated contributing guide's testing section.

Author: weiji14

docs/source/contributing.rst

@@ -130,8 +130,10 @@ Running the test suite
 ----------------------
 
 *cupy-xarray* uses the `pytest <https://docs.pytest.org/en/latest/contents.html>`_
-framework for testing. You can run the test suite using::
+framework for testing. You'll need to install some dependencies and then run the test
+suite like so::
 
+    pip install --group test
     pytest --doctest-modules cupy_xarray
 
 Contributing documentation

pyproject.toml

@@ -20,8 +20,12 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+
+[dependency-groups]
 test = [
     "dask",
+    "netcdf4",
+    "pooch",
     "pytest",
 ]