Plugwise

- Bugfix: implement solution for issue reported in [739](https://github.com/plugwise/plugwise-beta/issues/739)

- Prettying documents with Biome (CLI), fixture layout updated accordingly.

- Improve error handling, add group switching of plugs (Adam).

- Add missing typing for model_id.

- Initial support for Stretch (v2/v3) including tests
- Force gzip encoding, work-around for aiohttp-error
- Improve P1 DSMR legacy support
- Ensure `gateway_id` is properly defined (i.e. not `None`)
- b4: Use `domain_objects` over `direct_objects` endpoints
- Remove py3x internal modules (as requested per 86)
- CI-handling improvements and both 3.7 and 3.8 testing
- Code cleanup and output formatting improvements