Test a version of Trio that is compatible with AnyIO

[pquentin]

As suggested by @agronholm in #3089 (comment)

[agronholm]

Shouldn't we also add a lower bound for AnyIO? Some folks are still using 3.x from what I can see from the download charts.

[agronholm]

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.

[pquentin]

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

[agronholm]

Yup, makes perfect sense.