Deprecation of jsonschema.RefResolver

[jonblower]

Running validator with up-to-date versions of dependencies produces the following message:

covjson-validator/tools/validator.py:38: 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(None, referrer=None, store=schema_store)

So it would be good to have an update that removes the use of RefResolver