Update pygments from 2.3.0 to 2.3.1

Author: pyup-bot

dev-requirements.txt

@@ -13,7 +13,7 @@ pip_check_reqs==2.0.3
 pydocstyle==3.0.0  # Lint docstrings
 pyenchant==2.0.0  # Bindings for a spellchecking sytem
 pygithub==1.43.3
-Pygments==2.3.0
+Pygments==2.3.1
 pylint==2.2.2  # Lint
 pyroma==2.4  # Packaging best practices checker
 pytest-cov==2.6.0  # Measure code coverage