CI failing - All builds are failing on python 3.5 on pytest 4.6

[simonjayhawkins]

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=12138

cc @pandas-dev/pandas-core

[WillAyd]

Might be a result of the jump from pytest 4.5 to 4.6, though wasn't able to reproduce locally

[h-vetinari]

It is a pytest issue: pytest-dev/pytest#5358. Would help to pin "pytest<4.6" for now.

[jreback]

we can pin those builds specifically

[simonjayhawkins]

just about to submit PR, may have the syntax wrong though

# universal
- pytest>=4.0.2,!=4.6.0

[jreback]

use<4.6 (or for those older builds just pin directly to a single version