Format for unpack_from must be bytes. Closes #3533

celery/concurrency/asynpool.py

@@ -244,7 +244,7 @@ class ResultHandler(_pool.ResultHandler):
                            else EOFError())
                 Hr += n
 
-        body_size, = unpack_from('>i', bufv)
+        body_size, = unpack_from(b'>i', bufv)
         if readcanbuf:
             buf = bytearray(body_size)
             bufv = memoryview(buf)