Update pyyaml from 3.13 to 5.1

Author: pyup-bot

dev-requirements.txt

@@ -18,7 +18,7 @@ pylint==2.3.1  # Lint
 pyroma==2.4  # Packaging best practices checker
 pytest-cov==2.6.1  # Measure code coverage
 pytest==4.3.0  # Test runners
-PyYAML==3.13
+PyYAML==5.1
 Sphinx==1.8.5
 sphinx-autodoc-typehints==1.6.0
 sphinxcontrib-spelling==4.2.0