Update pluggy from 0.6.0 to 0.7.1

requirements.txt

 freezegun==0.3.10
 future==0.16.0
 numpy==1.15.0
-pluggy==0.6.0
+pluggy==0.7.1
 psutil==5.4.6
 py==1.5.4
 pyglet==1.3.2