Python-flask server stub generation fails using anyOf

Description

Example on a requestbody:

                    items: 
                      anyOf:
                        - $ref: '#/components/schemas/FingerPosition'
                        - $ref: '#/components/schemas/CameraPose'

this fails on python-flask (not on other server stubs) with the following error:

Error.There was an error generating the servers python-flask code. Please ensure that the definition and all external refs (inc. domains) are valid.

replacing with, for example:

                    items: 
                      type: string

works

Swagger-codegen version

Swagger Codegen to v. 2.4.9 for OpenAPI 2.0 and 3.0.13 for OpenAPI 3.0.

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement