Update flake8 from 3.5.0 to 3.7.1

Author: pyup-bot

requirements_dev.txt

@@ -13,7 +13,7 @@ quart==0.8.0;python_version>="3.7"  # pyup: update minor
 
 # tests
 coverage==4.5.1  # pyup: update minor
-flake8==3.5.0  # pyup: update minor
+flake8==3.7.1  # pyup: update minor
 tox==3.7.0  # pyup: update minor
 pytest==4.1.0  # pyup: update minor
 pytest-runner==4.2  # pyup: update minor