umqtt.simple: Add pubsub example.

umqtt.simple/example_pubsub.py

@@ -0,0 +1,13 @@
+import time
+from umqtt.simple import MQTTClient
+
+def sub_cb(topic, msg):
+    print((topic, msg))
+
+c = MQTTClient("uqmtt_client", "localhost")
+c.connect()
+c.subscribe(b"foo_topic")
+c.publish(b"foo_topic", b"hello")
+while 1:
+    c.wait_msg()
+c.disconnect()