convert shebang lines: python -> python3
diff --git a/Demo/turtle/tdemo_penrose.py b/Demo/turtle/tdemo_penrose.py
index f816f66..f73c864 100644
--- a/Demo/turtle/tdemo_penrose.py
+++ b/Demo/turtle/tdemo_penrose.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 """       xturtle-example-suite:
 
           xtx_kites_and_darts.py