[BUG] python-flask server ImportError exception at launch

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Exception has occurred: ImportError
cannot import name 'FlaskJSONEncoder' from 'connexion.apps' (/home/d3vopz/openapi/.venv/lib/python3.11/site-packages/connexion/apps/__init__.py)
  File "/home/d3vopz/openapi/test/resources/spec/out/python-flask-server/openapi/encoder.py", line 1, in <module>
    from connexion.apps import FlaskJSONEncoder
  File "/home/d3vopz/openapi/test/resources/spec/out/python-flask-server/openapi/__main__.py", line 5, in <module>
    from openapi import encoder
ImportError: cannot import name 'FlaskJSONEncoder' from 'connexion.apps' (/home/d3vopz/openapi/.venv/lib/python3.11/site-packages/connexion/apps/__init__.py)

openapi-generator version

7.0.1

OpenAPI declaration file content or url

Generation Details

api: python-flask server
pip3 install -r python-flask-server/requirements.txt

Steps to reproduce

curl api
unzip
pip3 install -r python-flask-server/requirements.txt
run in vscode

		"configurations": [
			{
				"name": "Python: oag",
				"type": "python",
				"request": "launch",
				"program": "/home/d3vopz/openapi/test/resources/spec/out/python-flask-server/openapi/__main__.py",
				"console": "integratedTerminal",
				"cwd": "/home/d3vopz/openapi",
				"env": {
					"PYTHONPATH": "/home/d3vopz/openapi/test/python/src:/home/d3vopz/etl/test/src:/home/d3vopz/openapi/test/resources/spec/out/python-flask-server"
				},
				"envFile": "/home/d3vopz/openapi/.env",
				"args":["--resource-path","/home/d3vopz/etl/resources/inject"]
			}
		]

cd /home/d3vopz/openapi ; /usr/bin/env /home/d3vopz/openapi/.venv/bin/python /home/d3vopz/.vscode-server/extensions/ms-python.python-2023.20.0/pythonFiles/lib/python/debugpy/adapter/../../debugpy/launcher 56187 -- /home/d3vopz/openapi/test/resources/spec/out/python-flask-server/openapi/__main__.py --resource-path /home/d3vopz/etl/resources/inject

Related issues/PRs

Suggest a fix

Start by reproducing the launch failure from test/resources/spec/out/python-flask-server using python-flask-server/requirements.txt, then inspect openapi/encoder.py and openapi/main.py, where the traceback points. Done means the generated server starts without the reported ImportError under the documented installation steps.