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

[T-256]

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...

Can you mention those lines? I think those lines are in httpcore.

OK I see there's a process of getting rid of anyio due to its performance penalty.

No, the main goal of this PR is to correct project specification when published. Since httpx codebase has not anyio then it's wrong to put it in project.dependencies .

[T-256]

solved in #2858