flake8: explicitly allow line break before or after binary operator (#9627)

W503 and W504 are issued for line breaks before or after binary
operators (respectively). This allows either approach (explicitly
instructing flake8 to ignore both of these cases).

.flake8

@@ -20,5 +20,5 @@
 # - N801: CapWords for class names.
 #
 [flake8]
-ignore = E129,E221,E241,E272,E731,F999,N801
+ignore = E129,E221,E241,E272,E731,F999,N801,W503,W504
 max-line-length = 79

.flake8_packages

@@ -18,5 +18,5 @@
 # - F821: undefined name `name` (needed for cmake, configure, etc.)
 #
 [flake8]
-ignore = E129,E221,E241,E272,E731,F999,F405,F821
+ignore = E129,E221,E241,E272,E731,F999,F405,F821,W503,W504
 max-line-length = 79