Ignore flake8 rule W503 (#2069)

By default, flake8 currently complains both when breaking before (W503)
and after (W504) a binary operator. A choice needs to be made!

Best practice evolved, around 2016, from breaking after to breaking before
a binary operator. Therefore ignore W503.

setup.cfg

@@ -3,4 +3,4 @@ addopts = --cov=codespell_lib -rs --cov-report= --tb=short
 
 [flake8]
 exclude = build, ci-helpers
-ignore =
+ignore = W503