Update pytest from 3.7.2 to 3.7.3

requirements.txt

 psutil==5.4.7
 py==1.5.4
 pyglet==1.3.2
-pytest==3.7.2
+pytest==3.7.3
 python-dateutil==2.7.3
 PyYAML==3.13
 redis==2.10.6