convert shebang lines: python -> python3
diff --git a/Demo/tkinter/guido/mbox.py b/Demo/tkinter/guido/mbox.py
index 45c384e..299999c 100755
--- a/Demo/tkinter/guido/mbox.py
+++ b/Demo/tkinter/guido/mbox.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python3
 
 # Scan MH folder, display results in window