Update pytest-cov from 2.5.1 to 2.6.0

Author: pyup-bot

requirements_dev.txt

@@ -20,7 +20,7 @@ pytest-runner==4.2
 pytest-asyncio==0.9.0
 pytest-tornado==0.5.0
 pytest-mock==1.10.0
-pytest-cov==2.5.1
+pytest-cov==2.6.0
 
 # docs
 Sphinx==1.7.8