Update pygments from 2.3.0 to 2.4.0 (#479)

Author: pyup-bot

requirements_dev.txt

@@ -24,6 +24,6 @@ pytest-cov==2.7.1  # pyup: update minor
 
 # docs
 Sphinx==2.0.0  # pyup: update minor
-Pygments==2.3.0  # pyup: update minor
+Pygments==2.4.0  # pyup: update minor
 sphinxcontrib-asyncio==0.2.0  # pyup: update minor
 sphinx-rtd-theme==0.4.2  # pyup: update minor