style: appease tidy.

components/style/properties/properties.mako.rs

@@ -901,7 +901,8 @@ impl LonghandIdSet {
     pub fn border_background_properties() -> &'static Self {
         ${static_longhand_id_set(
             "BORDER_BACKGROUND_PROPERTIES",
-            lambda p: (p.logical_group and p.logical_group.startswith("border")) or p.name in ["background-color", "background-image"]
+            lambda p: (p.logical_group and p.logical_group.startswith("border")) or \
+                       p.name in ["background-color", "background-image"]
         )}
         &BORDER_BACKGROUND_PROPERTIES
     }

servo-tidy.toml

@@ -89,6 +89,7 @@ files = [
   # Python 3 syntax causes "E901 SyntaxError" when flake8 runs in Python 2
   "./etc/taskcluster/decision_task.py",
   "./etc/taskcluster/decisionlib.py",
+  "./components/style/properties/build.py",
 ]
 # Directories that are ignored for the non-WPT tidy check.
 directories = [