Update validategeojson that used deprecated code

[gschwind]

Overview

Currently test fail with:

tests/validator/test_complexvalidators.py::ValidateTest::test_geojson_validator
/home/gschwind/projects/pywps/pywps/validator/complexvalidator.py:287: DeprecationWarning: jsonschema.RefResolver is deprecated as of v4.18.0, in favor of the https://github.com/python-jsonschema/referencing library, which provides more compliant referencing behavior as well as more flexible APIs for customization. A future release will remove RefResolver. Please file a feature request (on referencing) if you are missing an API for the kind of customization you need.
resolver = jsonschema.RefResolver(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

This patch update the code to follow Warning recommandation

Related Issue / Discussion

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[cehbrecht]

test failure not related to this PR