Yes, a PR would be better... also, the two versions of the patch appear to be identical?

CC'ing Antoine and Victor b/c they seem to have written this dangling_threads checking stuff and I'm not sure I understand it :-).

As some extra background: this is in Python's internal testing code, but @smurfix is hitting it because he's working on a third-party asyncio event loop implementation, and re-using test.asyncio to test it. I'm not sure what specifically is causing this to happen, though I guess it has something to do with how trio / trio-asyncio are using threads.

[1] https://github.com/python-trio/trio-asyncio