Update pytest-cov from 2.5.1 to 2.6.0

Author: pyup-bot

dev-requirements.txt

@@ -15,7 +15,7 @@ pydocstyle==2.1.1  # Lint docstrings
 pyenchant==2.0.0  # Bindings for a spellchecking sytem
 pylint==2.1.1  # Lint
 pyroma==2.4  # Packaging best practices checker
-pytest-cov==2.5.1  # Measure code coverage
+pytest-cov==2.6.0  # Measure code coverage
 pytest-envfiles==0.1.0  # Use files for environment variables for tests
 pytest==3.7.3  # Test runners
 timeout-decorator==0.4.0  # Decorate functions to time out.