Bug#969050: pytest-asyncio: AttributeError: type object 'Function' has no attribute 'from_parent' with our current pytest

[Rebecca N. Palmer]

This bug can happen even if the tests being run don't actually use 
pytest_asyncio (e.g. see pandas 1.0.5+dfsg-2); to avoid it one needs to 
not have it installed, i.e. remove the build/test-dependency.

This suggests that the current pytest-asyncio package is totally 
unusable without a more recent pytest.  And probably only even built 
because it doesn't run its own tests (arguably a bug in itself).