Update pytest from 3.5.1 to 3.6.2 (#675)

requirements.txt

@@ -12,7 +12,7 @@ pymongo==3.7.0
 pytest-cover==3.0.0
 pytest-randomly==1.2.3
 pytest-tornado==0.5.0
-pytest==3.5.1
+pytest==3.6.2
 python-dateutil==2.7.2
 pyyaml==4.1
 six==1.11.0