[bcp][worker] Decode job data if python older than 3.6

Prior to 3.6 the json module handles only str types. After that
more types are supported out of the box.

beat/core/bcp/worker.py

@@ -183,6 +183,8 @@ def run(
             if command == BCP.BCPE_EXECUTE:
                 job_id = request.pop(0)
                 job_data = request.pop(0)
+                if sys.version_info < (3, 6):
+                    job_data = job_data.decode("utf-8")
                 data = json.loads(job_data)
 
                 reply = [BCP.BCPP_JOB_RECEIVED, job_id]