wxGUI: fix legend in d.mon (https://trac.osgeo.org/grass/ticket/3896)

gui/wxpython/mapdisp/main.py

@@ -350,7 +350,7 @@ class LayerList(object):
     def __iter__(self):
         return self
 
-    def next(self):
+    def __next__(self):
         items = self._map.GetListOfLayers()
         try:
             result = items[self._index]
@@ -359,6 +359,9 @@ class LayerList(object):
         self._index += 1
         return result
 
+    def next(self):
+        return self.__next__()
+
     def GetSelectedLayers(self, checkedOnly=True):
         # hidden and selected vs checked and selected
         items = self._map.GetListOfLayers()