make __str__ call serialize() on nodes, c.f. bug #157872 Daniel

* python/libxml.py: make __str__ call serialize() on nodes, c.f.
  bug #157872
Daniel
diff --git a/python/libxml.py b/python/libxml.py
index 588e862..bde8aba 100644
--- a/python/libxml.py
+++ b/python/libxml.py
@@ -231,6 +231,8 @@
             self._o = _obj;
             return
         self._o = None
+    def __str__(self):
+        return self.serialize()
     def get_parent(self):
         ret = libxml2mod.parent(self._o)
         if ret == None: