diff --git a/sphinx/builders/linkcheck.py b/sphinx/builders/linkcheck.py
index b43849022..8079b680a 100644
--- a/sphinx/builders/linkcheck.py
+++ b/sphinx/builders/linkcheck.py
@@ -122,7 +122,10 @@ class CheckExternalLinksBuilder(Builder):
         # type: () -> None
         kwargs = {
             'allow_redirects': True,
-            'headers': {'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'},
+            'headers': {
+                'Accept': ('text/html,application/xhtml+xml,'
+                           'application/xml;q=0.9,*/*;q=0.8')
+            },
         }
         if self.app.config.linkcheck_timeout:
             kwargs['timeout'] = self.app.config.linkcheck_timeout