Pin pytest to 3.4.x

Closes #1170
diff --git a/requirements-tests.txt b/requirements-tests.txt
index b118ffd..b8db439 100644
--- a/requirements-tests.txt
+++ b/requirements-tests.txt
@@ -7,4 +7,4 @@
 coverage<4.0
 requests
 requests_mock
-pytest
+pytest>=3.4,<3.5