trim usage of six

[a-detiste]

This is artisanal manual craftwork :-)

     def mDone(self, button):
         if button == 0:        # "OK" or double click
-            if isinstance(self.tree.selection_key, six.string_types):
-                self.key = str(self.tree.selection_key)
-            else:
-                self.key = self.tree.selection_key
+            self.key = self.tree.selection_key

[joeraz]

Tested, no obvious issues. Merging.