Update sphinx from 1.8.3 to 1.8.4

Author: pyup-bot

dev-requirements.txt

@@ -20,7 +20,7 @@ pyroma==2.4  # Packaging best practices checker
 pytest-cov==2.6.1  # Measure code coverage
 pytest==4.1.1  # Test runners
 PyYAML==3.13
-Sphinx==1.8.3
+Sphinx==1.8.4
 sphinx-autodoc-typehints==1.6.0
 sphinxcontrib-spelling==4.2.0
 twine==1.12.1