Avoid importing/using anyio when trio backend is being used.

[tomchristie]

Somewhat prompted by #588 (comment)

When using trio, we currently use native trio for the backend implementation, but use anyio for some synchronisation primitives.

With a bit of careful thinking, and use of sniffio, we could re-jig those synchronisation primitives to avoid that, and ensure that...

• When running under asyncio, we use anyio for the network backend and synchronisation primitives.
• When running under trio, we use trio for the network backend and synchronisation primitives.

This would seem to me to be an improvement:

• We would no longer run into any clashes between anyio and trio versions.
• We'd be in a better position to move to installations with fewer dependancies. Potentially: 'pip install httpcore[asyncio]' and 'pip install httpcore[trio]'.