Add support for a Trio ConcurrencyBackend

[sethmlarson]

No description provided.

[tomchristie]

Indeed, yes.

I think there’s a conversation to be had about if that ends up in the core package or not.

My strong inclination is that we should support alternate concurrency backends, but that implementations other than asyncio should live in third party packages.

[florimondmanca]

Regarding whether alternative loops/IO libraries should be in third party packages, I think it makes sense in terms of relying on the community to support their own needs based on the reference implementation here.

One thing I think would be great though, is for httpx to provide a core test suite that alternatives could run their backend against. Don’t know exactly how that would materialize, but it would definitely allow to make sure that backends are compliant with the main API, as well as ease testing on the side of the third party package.