Use a more generic test for path item object's operations

Get the list of allowed operations from openapi_spec_validator:
https://github.com/p1c2u/openapi-spec-validator/blob/b2e439aa79c7cde932f9926f1ae124556924b10d/openapi_spec_validator/validators.py#L157

Author: pbasista

openapi_core/operations.py

@@ -4,6 +4,7 @@
 from functools import lru_cache
 
 from six import iteritems
+from openapi_spec_validator.validators import PathItemValidator
 
 from openapi_core.exceptions import InvalidResponse
 from openapi_core.parameters import ParametersGenerator
@@ -56,7 +57,7 @@ def __init__(self, dereferencer, schemas_registry):
     def generate(self, path_name, path):
         path_deref = self.dereferencer.dereference(path)
         for http_method, operation in iteritems(path_deref):
-            if http_method.startswith('x-') or http_method == 'parameters':
+            if http_method not in PathItemValidator.OPERATIONS:
                 continue
 
             operation_deref = self.dereferencer.dereference(operation)