convert shebang lines: python -> python3
diff --git a/Demo/tkinter/guido/electrons.py b/Demo/tkinter/guido/electrons.py
index e3bf468..6079c04 100755
--- a/Demo/tkinter/guido/electrons.py
+++ b/Demo/tkinter/guido/electrons.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python3
 
 # Simulate "electrons" migrating across the screen.
 # An optional bitmap file in can be in the background.