fix enum support in oneof/anyof in python-nextgen (#15340)

samples/client/echo_api/python-nextgen/openapi_client/models/string_enum_ref.py

@@ -13,7 +13,7 @@
 """
 
 
-from inspect import getfullargspec
+import json
 import pprint
 import re  # noqa: F401
 from aenum import Enum, no_arg
@@ -34,3 +34,9 @@ class StringEnumRef(str, Enum):
     FAILURE = 'failure'
     UNCLASSIFIED = 'unclassified'
 
+    @classmethod
+    def from_json(cls, json_str: str) -> StringEnumRef:
+        """Create an instance of StringEnumRef from a JSON string"""
+        return StringEnumRef(json.loads(json_str))
+
+