What also caught my eye was the explicit use of sniffio. Why is there a helper function _sleep() instead of just using anyio.sleep()? And the conftest.py use of sniffio could just be replaced by the use of the anyio_backend_name fixture. That would let you drop the explicit dependency on sniffio.

Good point! Initially anyio wasn't a runtime dependency, but then we needed a queue, and I did not clean up that part.

Yup, makes perfect sense.