In combination with `.aclose()`-ing the async gen instance returned from
`Portal.run()` this demonstrates the python bug:
https://bugs.python.org/issue32526
I've commented out the line that triggers the bug for now since this
case provides motivation for adding our own `trio.abc.ReceiveMemoryChannel`
implementation to be used instead of async gens directly (returned from
`Portal.run()`) since the latter is **not** task safe.