anyio/Trio compatibility

[smurfix]

This patch (OK, three of them) adds Trio compatibility, by way of anyio.

It also fixes two minor other issues, in separate patches:

• don't use iscoroutinefunction. That prevents wrapping async functions with a sync wrapper, which is perfectly legitimate Python code.
• don't compare to False with "==". Either use "not", if None, zero or the empty string should be considered false, or "is" if they should not be. I opted for the latter approach.

[smurfix]

I have to admit that I have no idea what I'm doing with the poetry.lock file … reverting that change. Somebody else (who knows their way around poetry …) can remove the dependency on pytest.asyncio; it's no longer needed.

[pnickolov]

Thank you @smurfix for the PR!
I think all we need to do is let poetry update the lock file with the new dependency. I think we can do it as part of the merge.

[smurfix]

Ah, OK, so why do you ship the poetry.lock file in the first place, given that it's autogenerated? IMHO that's generally not a good idea …

The tests fail because it couldn't find anyio 4.1, which is strange because that has been out for some time by now.