Support OpenAPI Specification 3.1.1

OAS 3.1.1 added
https://github.com/OAI/OpenAPI-Specification/blob/main/tests/v3.1/pass/non-oauth-scopes.yaml,
which indicates that it's possible for the `responses` object to be
missing; this caused a test failure.  It seems reasonable enough to
default to an empty dictionary.

Author: cjwatson

sphinxcontrib/openapi/openapi31.py

@@ -279,7 +279,7 @@ def _httpresource(
 ):
     # https://github.com/OAI/OpenAPI-Specification/blob/3.1.0/versions/3.1.0.md#operation-object
     parameters = properties.get("parameters", [])
-    responses = properties["responses"]
+    responses = properties.get("responses", {})
     query_param_examples = []
     indent = "   "