Update pytest-cov from 2.6.1 to 2.7.1

Author: pyup-bot

requirements/testing.txt

@@ -10,7 +10,7 @@ pytest==4.1.1  # pyup: <4.2
 pytest-django==3.4.6  # pyup: <3.4.7
 # pytest-xdist >1.27 requires pytest >4.1
 pytest-xdist==1.27.0  # pyup: ignore
-pytest-cov==2.6.1
+pytest-cov==2.7.1
 apipkg==1.5
 execnet==1.6.0