convert shebang lines: python -> python3
diff --git a/Demo/scripts/queens.py b/Demo/scripts/queens.py
index 726433c..ffd4bea 100755
--- a/Demo/scripts/queens.py
+++ b/Demo/scripts/queens.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python3
 
 """N queens problem.