Support sniffio

[Nathaniel J. Smith]

Now that sniffio is a thing, trio-asyncio should support it :-)

I think basically this means, making sure that the magic cvar is set to "asyncio" whenever trio-asyncio is running an asyncio callback.

[Matthias Urlichs]

done @ 0.8.2

[John Belmonte]

in general, please reference bugs from PR descriptions so we can follow the trail

[Nathaniel J. Smith]

@belm0 there was no PR, it was merged directly into master – that's what this discussion is about :-)

[John Belmonte]

As long as the bug # is in the commit description, github will link it as far as I'm aware (i.e. no PR needed).