convert shebang lines: python -> python3
diff --git a/Demo/turtle/tdemo_tree.py b/Demo/turtle/tdemo_tree.py
index fda653c..9c0b1f7 100644
--- a/Demo/turtle/tdemo_tree.py
+++ b/Demo/turtle/tdemo_tree.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 """      turtle-example-suite:
 
              tdemo_tree.py