Pin pytest on 3.x -- we arent ready for pytest 4.0 yet

Author: dpkp

tox.ini

@@ -9,7 +9,7 @@ log_format = %(created)f %(filename)-23s %(threadName)s %(message)s
 
 [testenv]
 deps =
-    pytest
+    pytest<4.0
     pytest-cov
     py{27,34,35,36,py}: pylint
     py{27,34,35,36,py}: pytest-pylint