Update pyflakes from 1.6.0 to 2.0.0 (#68)

Author: pyup-bot

requirements_dev.txt

@@ -8,7 +8,7 @@ flake8==3.6.0
 incuna-test-utils==6.6.0
 pip==18.1
 psycopg2-binary==2.7.5
-pyflakes==1.6.0
+pyflakes==2.0.0
 pycodestyle==2.4.0
 setuptools==40.4.3
 twine==1.12.1