Support checking anyio code too

[Zac-HD]

anyio is a compatibility layer on top of asyncio or Trio, allowing you to support trio-like structured concurrency using either. In practice, this means that most Trio-supporting libraries have a free way to support asyncio too, and I expect it'll be more generally helpful for the asyncio ecosystem as TaskGroups become more popular over the coming years.

Since almost all of flake8-trio's checks are actually about structured concurrency rather than being truly library-specific, and anyio matches the trio API quite closely, it would be relatively easy to add support for anyio. There are a couple of small divergences, but I think it would mostly be a matter of checking twice as many names.