undo: Use py3k-style super

Author: kaiw

meld/undo.py

@@ -84,7 +84,7 @@ def __init__(self, buffers):
         expects to maintain undo checkpoints for the same set of
         buffers for the lifetime of the UndoSequence.
         """
-        GObject.GObject.__init__(self)
+        super().__init__()
         self.buffer_refs = [weakref.ref(buf) for buf in buffers]
         self.clear()