convert shebang lines: python -> python3
diff --git a/Demo/tkinter/guido/MimeViewer.py b/Demo/tkinter/guido/MimeViewer.py
index 11701cd..b957c85 100755
--- a/Demo/tkinter/guido/MimeViewer.py
+++ b/Demo/tkinter/guido/MimeViewer.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python3
 
 # View a single MIME multipart message.
 # Display each part as a box.