Add anyio

[connorbrinton]

What is this project?

AnyIO is an asynchronous compatibility API that allows applications and libraries written against it to run unmodified on asyncio, curio and trio.

• GitHub: https://github.com/agronholm/anyio
• Docs: https://anyio.readthedocs.io/en/stable/

Why is it awesome?

• Provides high-level interface for writing asynchronous programs running on asyncio, curio and trio backends.
• Enables use of trio-like structured concurrency (task groups/nurseries) while using asyncio and curio.
• Includes additional features for cancellation, threads, signal handling, asynchronous file I/O, synchronization primitives and high-level networking.

[timofurrer]

Thanks for the suggestion @connorbrinton - we've already added it in another commit 🎉