HTTP response codes - cannot be like '4XX', must be integer or start with an integer

Whilst the openapi spec allows HTTP codes for responses objects to be of type 2XX or 4XX (link) indicating a range of codes, this sphinx contrib seems unable to use those response codes appearing in a spec, with a warning:

(ERROR/3) HTTP status code must be an integer (e.g. 200) or start with an integer (e.g. 200 OK); <#text: '4XX'> is invalid

Not sure if the issue originates in the sphinxcontrib.openapi or in a dependency.

Way to reproduce: use an openapi.yaml spec with something like this in its paths:

paths:
  /myendpoint:
    post:
      requestBody:
      ...
      responses:
        '4XX':
          description: ...
          content:
             ...

Reproduce the warning with the OpenAPI YAML example in the issue, then trace the response-code validation or rendering entry point that emits it. Compare the accepted keys with the OpenAPI 3.0.2 Responses Object rules. Done means response keys such as 4XX are accepted without the invalid-status warning.