Replace pytest-asyncio with anyio

[jacobtomlinson]

In other Dask projects we've found that pytest-asyncio causes a bunch of headaches. The test tooling from anyio is much more reliable.

This PR replaces pytest-asyncio with anyio as a test dependency. It configures anyio to only use the asyncio backend for testing and replaces the pytest marks where needed.

[jacobtomlinson]

Looks like we still need pytest-asyncio to be included in the environment.