refactor: replace deprecated validator interface

tests/test_schemas.py

-from openapi_spec_validator import openapi_v30_spec_validator
+from openapi_spec_validator import OpenAPIV30SpecValidator
 from openapi_spec_validator.readers import read_from_filename
 from openapi_schema_validator import validate
 from jsonschema.validators import RefResolver
@@ -58,7 +58,8 @@ class TestSpecs(SchemaTestCase):
         """
         print("TEST: compiled schema matches openapi v3 spec")
         schema = self.load_schema()
-        self.assertIsNone(openapi_v30_spec_validator.validate(schema))
+        errors = list(OpenAPIV30SpecValidator(schema).iter_errors())
+        self.assertEqual(len(errors), 0)
 
 
 class TestAllMessageExamples(SchemaTestCase):