[skip changelog] Resolve impossible to satisfy flake8 configuration (#1022)

Flake8/pycodestyle has a pair of mutually exclusive checks:

- W503: 	line break before binary operator
- W504: 	line break after binary operator

Having both these checks enabled results in a failed check when there is a line break at a binary operator that can't be
resolved by moving the operator.

PEP 8 recommends line break before the binary operator, so the logical choice between the two checks is W504.

.flake8

@@ -2,4 +2,8 @@
 
 [flake8]
 max-line-length = 120
-ignore = E741
+ignore =
+    E741,
+    # W503 and W504 are mutually exclusive, so one or the other must be ignored.
+    # PEP 8 recommends line break before, so we keep W504.
+    W503