build: allow binary operator after newline

It looks like they are currently forbidden both after and before
newlines: we choose the one that agrees with
https://www.flake8rules.com/rules/W503.html

> line breaks should occur before the binary operator because it keeps
> all operators aligned

.pre-commit-config.yaml

@@ -6,11 +6,11 @@ repos:
       args:
         - --in-place
         - --max-line-length=99
-        - --ignore=E402
+        - --ignore=E402,W503
     - id: flake8
       args:
         - --max-line-length=99
-        - --ignore=E402
+        - --ignore=E402,W503
     - id: check-ast
     - id: check-case-conflict
     - id: debug-statements