Remove RemovedInPytest4Warning resolved in latest pytest

[costrouc]

Resolved with issues: pytest-dev/pytest#4039

Addresses issue #90

[costrouc]

Here is one of the errors seems unrelated to this PR?

E       trio.TrioDeprecationWarning: trio.Event.clear is deprecated since Trio 0.12.0; use multiple Event objects or other synchronization primitives instead (https://github.com/python-trio/trio/issues/637)

[codecov]

Codecov Report

Merging #91 into master will decrease coverage by 1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   99.79%   98.79%   -1.01%     
==========================================
  Files          19       19              
  Lines         498      496       -2     
  Branches       43       43              
==========================================
- Hits          497      490       -7     
- Misses          1        5       +4     
- Partials        0        1       +1

Impacted Files	Coverage Δ
pytest_trio/_tests/conftest.py	100% <ø> (ø)	⬆️
pytest_trio/_tests/test_trio_asyncio.py	93.54% <100%> (-6.46%)	⬇️
pytest_trio/_tests/test_async_yield_fixture.py	97.61% <0%> (-2.39%)	⬇️
pytest_trio/plugin.py	98.51% <0%> (-1%)	⬇️

[pquentin]

Thanks! Unfortunately you got hit by python-trio/trio-asyncio#61, I added a commit to ignore the issue for now even if ideally we should fix it in trio-asyncio, and release trio-asyncio. (@oremanj actually made great progress on trio-asyncio lately.)

There's a PyPy issue left (we should probably upgrade PyPy) and a attrs version error that I don't understand with Python 3.5.2.