Update pytest from 3.6.3 to 3.7.1

requirements.txt

 psutil==5.4.6
 py==1.5.4
 pyglet==1.3.2
-pytest==3.6.3
+pytest==3.7.1
 python-dateutil==2.7.3
 PyYAML==3.13
 redis==2.10.6