flake8: disable the new checks of flake8 3.6 for now

(cherry picked from commit fc4a7bc0c90a1790ef83e9a4e6acf2b8422917f4)

setup.cfg

@@ -5,7 +5,8 @@ python_files = testsuite/*.py
 # please note that the values are adjusted so that they do not cause failures
 # with existing code. if you want to change them, you should first fix all
 # flake8 failures that appear with your change.
-ignore = E122,E123,E125,E126,E127,E128,E226,E402,E722,E731,E741,F401,F405,F811
+ignore = E122,E123,E125,E126,E127,E128,E226,E402,E722,E731,E741,F401,F405,F811,
+         W504,W605,F841
 # line length long term target: 120
 max-line-length = 255
 exclude = build,dist,.git,.idea,.cache,.tox,docs/conf.py