Remove pytest warnings and fix CI

[pquentin]

Closes #90, Closes #91

We no longer have the trio-asyncio problem, but had to bump PyPI and stop testing Python 3.5.2 because pytest fails to start with it. I also added entries for more recent Python versions.

[codecov]

Codecov Report

Merging #92 into master will decrease coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   99.79%   99.39%   -0.41%     
==========================================
  Files          19       18       -1     
  Lines         498      495       -3     
  Branches       43        0      -43     
==========================================
- Hits          497      492       -5     
- Misses          1        3       +2     

Impacted Files	Coverage Δ
pytest_trio/_tests/conftest.py	100.00% <ø> (ø)
pytest_trio/_tests/test_fixture_names.py	100.00% <100.00%> (ø)
pytest_trio/plugin.py	99.04% <100.00%> (-0.47%)	⬇️
pytest_trio/_tests/test_async_yield_fixture.py	97.67% <0.00%> (-2.33%)	⬇️
pytest_trio/_tests/test_basic.py	100.00% <0.00%> (ø)
pytest_trio/_tests/test_trio_mode.py	100.00% <0.00%> (ø)
pytest_trio/_tests/test_contextvars.py	100.00% <0.00%> (ø)
pytest_trio/_tests/test_sync_fixture.py	100.00% <0.00%> (ø)
pytest_trio/_tests/test_async_fixture.py	100.00% <0.00%> (ø)
... and 6 more

[pquentin]

Hmm, the Windows builds are full of failures, and it's surprising that we get coverage for all installed packages for Python 3.8 in Travis. So maybe that's a start to understand the coverage problem here.

[oremanj]

Remaining coverage changes will be fixed by #94. Going to merge this to unblock downstream projects from updating to support 0.15.0.