[BUG] [python-flask] Hosted UI doesn't match the openapi.json, missing readOnly information.

Description

It seems like readOnly fields get dropped from the Swagger UI after generating the server stub.

The spec downlaoded from the server at /api/v3/openapi.json still holds the readOnly field, and it shows correctly in the public https://editor.swagger.io/ , but the Swagger UI hosted by the generated server doesn't respect it.

openapi-generator version

4.3.1

OpenAPI declaration file content or url

Start with https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml
and set the following property:

    components:
      schemas:
        Pet:
          properties:
            id:
              readOnly: true

Steps to reproduce & generate

1. Modify the above petstore.yaml file
2. Generate a python-flask server: docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli:v4.3.1 generate --input-spec /local/petstore.yaml --generator-name python-flask --output /local
3. Build & Run: docker build -t openapi_server ., docker run -p 8080:8080 openapi_server
4. Visit http://localhost:8080/api/v3/ui/#/pet/add_pet. Switch to application/x-www-form-urlencoded. The id field is no longer readOnly- it appears in the Request Body Parameters.
5. Download the /api/v3/openapi.json file from the UI, it still has the readOnly field, and it displays OK if pasted into https://editor.swagger.io/

Generate the modified petstore.yaml with the python-flask generator and inspect the hosted Swagger UI at /api/v3/ui/#/pet/add_pet. Compare the application/x-www-form-urlencoded request fields with /api/v3/openapi.json and the same specification in editor.swagger.io; done means the readOnly id field is treated consistently in the generated UI.