Un-ignore some linting rules

setup.cfg

 [flake8]
 max-line-length = 120
 exclude=build,.eggs,.git
+# Flake8 ignores multiple errors by default;
+# the only interesting ignore is W503, which goes against PEP8.
+# See https://lintlyci.github.io/Flake8Rules/rules/W503.html
+ignore=W503