Make the Python SDK fully async on httpx2 - #173

The README error-handling example references a users variable that is local to an earlier function and raises NameError when run.

This PR migrates the Python SDK transport and service APIs to asynchronous httpx2/anyio, adds explicit client-pool lifecycle management, and updates chunked uploads for async progress callbacks. It also updates tests, package metadata, and documentation for the async API and Python 3.11+ requirement.