Address warning from jsonschema 4.18.x

[allanlewis]

As of v4.18.0, jsonschema emits the following warning related to jsonschema.exceptions.RefResolutionError:

DeprecationWarning: jsonschema.exceptions.RefResolutionError is deprecated as of version 4.18.0. If you wish to catch potential reference resolution errors, directly catch referencing.exceptions.Unresolvable.

It would be good if this package could address this warning before RefResolutionError is removed in some future version.

[allanlewis]

In ignoring this warning in my project (I set PYTHONWARNINGS=error when running unit tests) I then get another warning:

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.

So there might be a chain of deprecations to handle here 😕

[allanlewis]

These changes are documented in the release notes for jsonschema 4.18.0: https://github.com/python-jsonschema/jsonschema/releases/tag/v4.18.0