[Python] NameError: name 'Category' is not defined from the generated flask server

Description

When I run flask-server, I get bellow ImportError.

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/lib/python3.8/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/usr/src/app/swagger_server/__main__.py", line 5, in <module>
    from swagger_server import encoder
  File "/usr/src/app/swagger_server/encoder.py", line 4, in <module>
    from swagger_server.models.base_model_ import Model
  File "/usr/src/app/swagger_server/models/__init__.py", line 9, in <module>
    from swagger_server.models.pet import Pet
  File "/usr/src/app/swagger_server/models/pet.py", line 12, in <module>
    class Pet(Model):
  File "/usr/src/app/swagger_server/models/pet.py", line 18, in Pet
    def __init__(self, id: int=None, category: Category=None, name: str=None, photo_urls: List[str]=None, tags: List[Tag]=None, status: str=None):  # noqa: E501
NameError: name 'Category' is not defined

Swagger-codegen version

2.4.13

Swagger declaration file content or url

https://petstore.swagger.io/v2/swagger.json

Command line used for generation

I generate from swagger-editor.

Steps to reproduce

docker build -t swagger_server .
docker run -p 8080:8080 swagger_server

Related issues/PRs

nothing

Suggest a fix/enhancement

I can fix this bug when adding two line in ./swagger_server/models/pet.py .

  from swagger_server.models.base_model_ import Model
+ from swagger_server.models.category import Category
+ from swagger_server.models.tag import Tag
  from swagger_server import util

Reproduce the problem with the provided Docker commands and inspect the generated swagger_server/models/pet.py, especially its imports. Confirm that Category and Tag are available when the generated Flask server starts, then verify the same container startup path completes without the NameError.