Support Python 3.11

## Request

The AWS SDK for Python currently requires Python 3.12 or newer. This issue tracks adding support for Python 3.11, which remains supported upstream until October 2027. It follows the version-support discussion in #8. Python 3.10 is outside this issue's scope.

Our initial review indicates that Python 3.11 support is feasible, but we have not committed to a release or timeline. If this affects you, please add a thumbs-up reaction. Comments describing blocked use cases or deployment environments will help us understand the impact.

## Required Changes

Generated service-client code already appears compatible with Python 3.11. Most changes are needed in the Smithy runtime:

- Replace Python 3.12-only PEP 695 syntax with `TypeAlias`, `TypeVar`, and `Generic`
- Use `typing_extensions.override` on Python 3.11
- Update runtime and code-generator metadata
- Add Python 3.11 CI and test coverage
- Regenerate affected clients
- Test aiohttp, AWS CRT, and streaming operations
- Update SDK documentation and classifiers

The third-party transport and JSON parser dependencies we reviewed already support Python 3.11.

Most implementation work will take place in [smithy-lang/smithy-python](https://github.com/smithy-lang/smithy-python). This issue will track the SDK release and provide a place to follow progress.

## Completion Criteria

- Generated clients install and import on Python 3.11
- Unit, protocol, transport, and streaming tests pass
- Published packages declare Python 3.11 support
- Documentation and CI include Python 3.11