[BUG][Python-server] Typings not working correctly

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

PYTHON

AbstractCodegenGenerator for Connexion WebServer is gernerating invalid code.
In a specific use case.
If a class is referencing its type in the init function without single quotes.
https://stackoverflow.com/questions/35617608/type-hinting-for-objects-of-type-thats-being-defined

NameError: name 'TreeGroup' is not defined

openapi-generator version

• 4.0.0

OpenAPI declaration file content or url

components
  schemas:
    TreeGroup:
      properties:
        treeGroupId:
          readOnly: true
          type: string
        name:
          type: string
        groups:
          type: array
          items:
            $ref: "#/components/schemas/TreeGroup"
      required:
        - name
        - groups
      type: object

Command line used for generation

java -DsupportingFiles=util.py,__init__.py,base_model_.py -Dmodels -DskipFormModel=true -jar openapi-generator-cli-4.0.0.jar generate -i openapi-v3.yaml -g python-aiohttp -o ./out/python --skip-validate-spec -c python-config.json --model-package dataobjects

Steps to reproduce

Generate a YAML with an components.schemas as above and generate the models.
This is a self reference.
Try to import them into a python shell.

from openapi_server.dataobjects import TreeGroup

Related issues/PRs

• <->

Suggest a fix

This approach fixed the bug.

https://stackoverflow.com/questions/35617608/type-hinting-for-objects-of-type-thats-being-defined

class TreeGroup(Model):
    def __init__(self, tree_group_id: str=None, name: str=None, groups: List[TreeGroup]=None):

class TreeGroup(Model):
    def __init__(self, tree_group_id: str=None, name: str=None, groups: List['TreeGroup']=None):

Think this line is responsible for the output but don't know it for sure.

https://github.com/OpenAPITools/openapi-generator/blob/63e1e0451c61d7eb5f191c54d5e68f5fb02f5994/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonAbstractConnexionServerCodegen.java#L293-L303

Start at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonAbstractConnexionServerCodegen.java#L293-L303 and reproduce the issue with the supplied OpenAPI schema and generation command. Confirm the generated self-referential TreeGroup model can be imported successfully and uses the expected quoted type reference.