fixes Object of type date is not JSON serializable error

[bgunebakan]

The to_json function in the Python generator currently throws a TypeError when attempting to serialize objects that contain date types such as datetime.date. This issue affects all Python clients generated by the OpenAPI generator where the schema contains date types. The error encountered is: "Object of type date is not JSON serializable."

Related Issue: #18397

Proposed Changes:
This update modifies the to_json method in the model_generic.mustache template to incorporate a more dynamic approach to excluding fields during serialization. This approach ensures that fields that could potentially cause serialization errors, such as read-only fields or additional properties, are automatically excluded based on the model configuration.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

thanks for the PR. please follow step 3 to update the samples so that the CI can verify the change.

[wing328]

cc
@cbornet (2017/09) @tomplus (2018/10) @krjakbrjak (2023/02) @fa0311 (2023/10) @multani (2023/10)

[wing328]

the python tests failed.

can you please take a look when you've time?