fix create_spec import failure

Author: pulak-opti

tests/acceptance/helpers.py

@@ -4,7 +4,7 @@
 from random import randint, choice
 
 import yaml
-from openapi_core import create_spec
+from openapi_core.spec import create_spec
 from openapi_core.validation.request.datatypes import (OpenAPIRequest, RequestParameters)
 from openapi_core.validation.request.validators import RequestValidator
 from openapi_core.validation.response.datatypes import OpenAPIResponse