Fix new visitchild() transform helper method.

Cython/Compiler/Visitor.pxd

-# cython: language_level=3
+# cython: language_level=3str
 
 cimport cython
 

Cython/Compiler/Visitor.py

@@ -279,10 +279,11 @@ class VisitorTransform(TreeVisitor):
     def visitchild(self, parent, attr, idx=0):
         # Helper to visit specific children from Python subclasses
         child = getattr(parent, attr)
-        if attr is not None:
+        if child is not None:
             node = self._visitchild(child, parent, attr, idx)
             if node is not child:
                 setattr(parent, attr, node)
+            child = node
         return child
 
     def recurse_to_children(self, node):