Update pytest from 3.8.2 to 3.9.2

Author: pyup-bot

dev-requirements.txt

@@ -17,7 +17,7 @@ Pygments==2.2.0
 pylint==2.1.1  # Lint
 pyroma==2.4  # Packaging best practices checker
 pytest-cov==2.6.0  # Measure code coverage
-pytest==3.8.2  # Test runners
+pytest==3.9.2  # Test runners
 PyYAML==3.13
 Sphinx==1.8.1
 sphinx-autodoc-typehints==1.3.0