Update pytest-cov from 2.8.1 to 2.9.0 (#349)

Author: pyup-bot

requirements-testing.txt

@@ -2,7 +2,7 @@ python-coveralls==2.9.3
 coverage==5.1
 flake8==3.8.1
 pytest==5.4.2
-pytest-cov==2.8.1
+pytest-cov==2.9.0
 autopep8==1.5.2
 mypy==0.770
 py-spy==0.3.3