Update pytest from 3.7.3 to 3.8.0

Author: pyup-bot

dev-requirements.txt

@@ -17,7 +17,7 @@ pylint==2.1.1  # Lint
 pyroma==2.4  # Packaging best practices checker
 pytest-cov==2.5.1  # Measure code coverage
 pytest-envfiles==0.1.0  # Use files for environment variables for tests
-pytest==3.7.3  # Test runners
+pytest==3.8.0  # Test runners
 timeout-decorator==0.4.0  # Decorate functions to time out.
 vulture==0.29
 yapf==0.23.0  # Automatic formatting for Python