Update pytest from 3.4.2 to 3.5.1

Author: pyup-bot

dev-requirements.txt

@@ -16,7 +16,7 @@ pylint==1.8.3  # Lint
 pyroma==2.3  # 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==3.4.2  # Test runners
+pytest==3.5.1  # Test runners
 timeout-decorator==0.4.0  # Decorate functions to time out.
 vulture==0.26
 yapf==0.21.0  # Automatic formatting for Python