Fix yamllinter handling of tuple assignment.

test/sanity/yamllint/yamllinter.py

@@ -120,6 +120,9 @@ class YamlChecker(object):
         def check_assignment(statement, doc_types=None):
             """Check the given statement for a documentation assignment."""
             for target in statement.targets:
+                if isinstance(target, ast.Tuple):
+                    continue
+
                 if doc_types and target.id not in doc_types:
                     continue