convert shebang lines: python -> python3
diff --git a/Demo/sockets/mcast.py b/Demo/sockets/mcast.py
index b7f8556..6ce7c6d 100755
--- a/Demo/sockets/mcast.py
+++ b/Demo/sockets/mcast.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Send/receive UDP multicast packets.
 # Requires that your OS kernel supports IP multicast.