Update python-dateutil from 2.7.2 to 2.7.3

requirements.txt

 py==1.5.3
 pyglet==1.3.2
 pytest==3.5.1
-python-dateutil==2.7.2
+python-dateutil==2.7.3
 PyYAML==3.12
 redis==2.10.6
 six==1.11.0