Update Python client generator

The current Python client generator is a little bit buggy and features cumbersome models.

There is a new version featuring Pydantic models.
in v6.6.0 stable release, the generator name is `python-nextgen`, will be the default from 7.

- [ ] try to generate api client code for sample app
- [ ] test it
- [ ] If it works, update the client generator to use this as default
- [ ] Evaluate replacement of cloudharness models with Pydantic models

Start by using the proposed python-nextgen generator at version 6.6.0 to generate client code for the sample app, then test the generated client. Compare the generated models with the existing cloudharness models and evaluate whether Pydantic models are suitable. Done means the generator works for the sample app and a decision is made about making it the default.

Written by the indexing model from the issue text.