[BUG] [python-flask] member variable incorrect

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

member variable incorrect

openapi-generator version

v4.3.0

OpenAPI declaration file content or url

openapi: 3.0.0

info:
  version: "1.0.0"
  title: "Test"

paths:
  /in:
    get:
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/C'
components:
  schemas:
    A:
      type: object
      properties:
        a:
          type: string
    B:
      type: object
      properties:
        b:
          type: string
      allOf:
      - $ref: '#/components/schemas/A'
    C:
      type: object
      properties:
        c:
          type: string
      allOf:
      - $ref: '#/components/schemas/B'

Command line used for generation

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli:v4.3.0 generate -i /local/x.yaml -g python-flask -o /local/out/python -Dmodels

Steps to reproduce

1. save spec file as x.yaml in current directory
2. run command above
3. cat out/python/openapi_server/models/c.py only member variable a/c in class C
4. paste spec file into http://editor.swagger.io/ all member variable a/b/c in class C

Related issues/PRs

Maybe the same issue? https://github.com/OpenAPITools/openapi-generator/issues/927

Suggest a fix

Reproduce the issue with the provided OpenAPI YAML and the python-flask generator command, then inspect the generated out/python/openapi_server/models/c.py. Compare class C with the inherited properties shown in Swagger Editor, especially a, b, and c. Done means the generated C model includes all inherited member variables and the relevant regression behavior is covered.