Drop anyio dependency

[T-256]

It is currently used indirectly by httpcore's asyncio feature.
There is no usage of anyio in httpx's codebase.

[rafalkrupinski]

There's some code that tries to use trio and falls back to asyncio, like Event or sleep. Why not use anyio for this? Less code to maintain...

edit:
OK I see there's a process of getting rid of anyio due to its performance penalty.
I'm just gonna link this: agronholm/anyio#736