Update Python client generator

[filippomc]

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