Must treat 3.x and 2.7 separately.

Change-Id: Idcea1dcd2b22b757d467b0ad5478b48a515afc73

src/paho/mqtt/client.py

@@ -840,7 +840,9 @@ class Client(object):
             raise ValueError('Invalid topic.')
         if qos<0 or qos>2:
             raise ValueError('Invalid QoS level.')
-        if isinstance(payload, str) or isinstance(payload, bytearray) or isinstance(payload, unicode):
+        if isinstance(payload, str) or isinstance(payload, bytearray):
+            local_payload = payload
+        elif sys.version_info[0] < 3 and isinstance(payload, unicode):
             local_payload = payload
         elif isinstance(payload, int) or isinstance(payload, float):
             local_payload = str(payload)