extend() doesn't copy applicable_validators

[PetrusAsikainen]

It seems the applicable_validators property from c6b0fc4 never made it to extend.

This means that extending Draft4Validator (or other legacy validators) makes it consider properties next to $ref again. (This made connexion break OpenAPI 3.0 spec - spec-first/connexion#1721)

Steps to reproduce

import jsonschema

schema = {
  "$defs": { "test": { "type": "number" } },
  "$ref": "#/$defs/test",
  "maximum": 1
}

# no error, maximum is ignored as per spec
jsonschema.validate(schema=schema, instance=2, cls=jsonschema.Draft4Validator)

# extend() with no changes
BadValidator = jsonschema.validators.extend(jsonschema.Draft4Validator)

# jsonschema.exceptions.ValidationError: 2 is greater than the maximum of 1
jsonschema.validate(schema=schema, instance=2, cls=BadValidator)

Environment

$ python --version
Python 3.11.2
$ pip show jsonschema | grep Version
Version: 4.18.2

[Julian]

Should be released in just a bit in v4.18.3. Thanks.