Update pytest-cov from 2.6.1 to 2.7.1

Author: pyup-bot

dev-requirements.txt

@@ -16,7 +16,7 @@ pygithub==1.43.7
 Pygments==2.3.1
 pylint==2.3.1  # Lint
 pyroma==2.4  # Packaging best practices checker
-pytest-cov==2.6.1  # Measure code coverage
+pytest-cov==2.7.1  # Measure code coverage
 pytest==4.4.1  # Test runners
 PyYAML==5.1
 Sphinx==2.0.1