Update pytest-cov from 2.7.1 to 2.8.1

Author: pyup-bot

dev-requirements.txt

@@ -17,7 +17,7 @@ pygithub==1.43.8
 Pygments==2.4.2
 pylint==2.4.2  # Lint
 pyroma==2.5  # Packaging best practices checker
-pytest-cov==2.7.1  # Measure code coverage
+pytest-cov==2.8.1  # Measure code coverage
 pytest==5.2.0  # Test runners
 PyYAML==5.1.2
 Sphinx==2.2.0