[#221] Update pluggy version to appease pytest

Based off discussion [here](pytest-dev/pytest#3753)

Author: PeterGhimself

requirements.txt

@@ -6,7 +6,7 @@ kiwisolver==1.0.1
 matplotlib==2.2.2
 more-itertools==4.2.0
 numpy==1.14.5
-pluggy==0.6.0
+pluggy>=0.7
 py==1.5.4
 #pycairo==1.17.1
 #PyGObject==3.28.3 # unfortunately the version that Trusty supports for python3-gi is too outdated for pyobject