QPID-8631: use `next()` to progress the iterator in dom.py, so that setup.py runs
diff --git a/mllib/dom.py b/mllib/dom.py
index 4762087..49ba0d0 100644
--- a/mllib/dom.py
+++ b/mllib/dom.py
@@ -244,7 +244,7 @@
     sources = [iter(self.source)]
     while sources:
       try:
-        nd = sources[-1].next()
+        nd = next(sources[-1])
         if isinstance(nd, Tree):
           sources.append(iter(nd.children))
         else: