WIP: async support

[matthewelwell]

This PR adds async support to the python client.

Note that this is a draft at the moment, and takes the approach of simply replacing the current client with one that supports async. Probably to get this into a releasable state, we want to either create async / non-async methods on the client, or create 2 separate clients - an async and a non-async one.

This PR is also a breaking change, as it replaces the use of the requests module with the httpx module. As such, we will no longer be able to support the Retry class that is currently provided as an argument to the constructor.