Unpin pytest - not sure why it was pinned.

tests/requirements.txt

@@ -3,5 +3,5 @@ mock
 pytest-flake8; python_version>="2.7"
 virtualenv>=13.0.0
 pytest-virtualenv>=1.2.7
-pytest>=3.0.2,<=3.2.5
+pytest>=3.0.2
 wheel