[BUG] [Python] to_json function raise error on date type

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

[BUG] [Python] to_json function raise error on date type: TypeError: Object of type date is
not JSON serializable

openapi-generator version

7.4

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: API
  description: API
  version: 1.4.0
paths:
  /create:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/Pet"
      responses:
        "202":
          description: OK
components:
  schemas:
    Pet:
      type: object
      properties:
        asOfDate:
          format: date
          type: string
        color:
          type: string

Generation Details

Steps to reproduce

1. generate sdk with the spec and version 7.4
2. use the code below to test it

pet_dict = {
        "color": "blue",
        "asOfDate": "2020-03-03"
    }
    pet = SDK.Pet.from_json(json.dumps(pet_dict))
    print(pet)
    // error: Object of type date is 
not JSON serializable
    pet.to_json()

Related issues/PRs

Suggest a fix

Start with the generated Python SDK model's Pet.from_json and to_json behavior using the OpenAPI declaration and reproduction steps in the issue. Confirm that a Pet with an asOfDate date value can be serialized without the reported TypeError, and verify the resulting JSON output.