Update pytest-cov from 2.6.1 to 2.7.1

Author: pyup-bot

requirements-dev.txt

@@ -9,7 +9,7 @@ docker==4.0.1
 flake8==3.7.7
 ipdb==0.12
 motor==2.0.0
-pytest-cov==2.6.1
+pytest-cov==2.7.1
 pytest-sugar==0.9.2
 pytest==4.6.1
 python-dateutil==2.8.0