Drop anyio dependency

[miili]

Hi @alexdelorenzo,

thanks for this handy library. Delving into the code I cannot see the necessity for anyio dependency.

stdlib asyncio provides to_thread or loop.run_in_executor to push sync functions to threads.
What is the motivation to using AnyIO?

Maybe aiofiles can be demoted to dev dependency, as it is only used by tests. To clean the installation procedure we could remove setup.py and requirements*.txt to use only pyproject.toml / poetry.

Would a PR be welcome?

Thanks and merry christmas!

M