convert shebang lines: python -> python3
diff --git a/Demo/tkinter/guido/dialog.py b/Demo/tkinter/guido/dialog.py
index 1832ba4..f16029c 100755
--- a/Demo/tkinter/guido/dialog.py
+++ b/Demo/tkinter/guido/dialog.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python3
 
 # A Python function that generates dialog boxes with a text message,
 # optional bitmap, and any number of buttons.