* python: type generated client using Self
This doesn't offer a clear win but this helps for:
* Using modern types and making the typing intent clearer
* Decreasing the need for `from __future__ import annotations`, since a
class can now refer to itself without using its name
* Using more `cls` to automatically refer to the class, instead of
respecifying the class name every time
Self is available from Python 3.11 and is provided in typing_extensions
(since 4.0.0) as a fallback for older versions
See: https://peps.python.org/pep-0673/
See: https://github.com/python/typing_extensions/blob/main/CHANGELOG.md#added-in-version-400
* generate code
