[BUG] [Python] $ref references to non-object types not working correctly

Created by: spacether

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

When looking at the swagger2.0 sample spec, the #/definitions/OuterEnum defines a string with enum examples. When it is used in other models, the parameters that refer to OuterEnum with $ref incorrectly list themselves as model parameters and do not include the correct enum options.

openapi-generator version

4.0.0-SNAPSHOT

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml

definitions:
  OuterEnum:
    type: string
    enum:
    - "placed"
    - "approved"
    - "delivered"
  Enum_Test:
    type: object
    required:
      - enum_string_required
    properties:
      enum_string:
        type: string
        enum:
          - UPPER
          - lower
          - ''
      enum_string_required:
        type: string
        enum:
          - UPPER
          - lower
          - ''
      enum_integer:
        type: integer
        format: int32
        enum:
          - 1
          - -1
      enum_number:
        type: number
        format: double
        enum:
          - 1.1
          - -1.2
      outerEnum:
        $ref: '#/definitions/OuterEnum'

Command line used for generation

java -DdebugModels -jar /Users/justin/programming/openapi/fork_mine/openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -t modules/openapi-generator/src/main/resources/python -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g python -o samples/client/petstore/python -DpackageName=petstore_api

Steps to reproduce

Run the above debugging for the models
Inspect the OuterEnum model result, its dataType='str'
Inspect the Enum_Test OuterEnum parameter, isModel is true and none of the enum values have been connected in

Related issues/PRs

Suggest a fix

For models where datatype is not object, we should:

Convert them to codegen parameters
Put them where they belong
Remove the source models from the final models list