Revert "pin pytest to disallow 2.8.4 which breaks tests"

This reverts commit ed48066a3f8e58af58926a17906540213e57f88e.
diff --git a/tox.ini b/tox.ini
index cf899b7..63f1e4b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,7 +7,7 @@
     coverage
     iso8601
     pretend
-    pytest!=2.8.4
+    pytest
     hypothesis>=1.11.4
     pyasn1_modules
     ./vectors