tox: Avoid installing pytest==5.2.3 due to an introduced regression

This will ensure that others don't git bitten by accidentally picking up
`pytest==5.2.3`.

See https://github.com/pytest-dev/pytest/issues/6194.

tox.ini

@@ -5,7 +5,7 @@ envlist = py27,py34,py35,py36,py37,py38,flake8,linters,docs
 [testenv]
 deps =
     mock>=2.0.0
-    pytest!=3.0.5
+    pytest!=3.0.5,!=5.2.3
     coverage
 commands =
     coverage run -m pytest {posargs}