Use python3 for gtk-dialog.py

As mentioned in #20059 (comment)

packaging/linux/gtk-dialog.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 """A simple GTK3 graphical dialog helper that can be used as a fallback if zenity is not available
    Compatible with python 2 or 3 with the gi bindings.