Add python3.9 compatiblity

Starting with python3.9, launching of mugshot will fail,
as getiterator() been deprecated since Python 2.7,
and has been removed in Python 3.9. Using iter() will fix that.

Signed-off-by: Conrad Kostecki <conrad@kostecki.com>

mugshot_lib/Builder.py

@@ -89,7 +89,7 @@ def add_from_file(self, filename):
         tree = ElementTree()
         tree.parse(filename)
 
-        ele_widgets = tree.getiterator("object")
+        ele_widgets = tree.iter("object")
         for ele_widget in ele_widgets:
             name = ele_widget.attrib['id']
             widget = self.get_object(name)
@@ -111,7 +111,7 @@ def add_from_file(self, filename):
             if connections:
                 self.connections.extend(connections)
 
-        ele_signals = tree.getiterator("signal")
+        ele_signals = tree.iter("signal")
         for ele_signal in ele_signals:
             self.glade_handler_dict.update(
                 {ele_signal.attrib["handler"]: None})