* Support aiohttp handlers to return tuples
* Minor update from #828 review
* Factorize more code between Flask and AioHttp response
* Fix CI
* Drop six string types
* Standardize response logging
* Handle one-tuples that only contain data
* clean up a couple of type hint comments
* Add a few more get_response tests
* Adjust _prepare_body interface to simplify improving _serialize_data
Rename _jsonify_data to _serialize_data to make its purpose easier to
understand (this was also known as _cast_body in aiohttp_api).
In exploring how to harmonize json serialization between aiothttp and
flask, we needed to be able to adjust the mimetype from within
_serialize_data. Harmonizing the actual serialization has to wait until
backwards incompatible changes can be made, but we can keep the new
interface, as these functions were introduced in this PR (#849).
* Add deprecation warnings about implicit serialization
