Add python version checks and 3.4 example

contrib/zmq/zmq_sub.py

@@ -11,6 +11,11 @@ import binascii
 import asyncio, zmq, zmq.asyncio
 import signal
 import struct
+import sys
+
+if not (sys.version_info.major >= 3 and sys.version_info.minor >= 5):
+    print("This example only works with Python 3.5 and greater")
+    exit(1)
 
 port = 28332