Fix issue: expect redis pubsub data to be str type instead of bytes type (sonic-net#196)

**- What I did**

Change redis pubsub expect data type from bytes to str. There was a snmpagent framework change which transfer redis pubsub data from bytes to str, after that change, there is no need for non-framework code to do this transfer. So now we need handle str data directly.

**- How I did it**

Change data expectation from bytes to str

**- How to verify it**

Run regression and manual test

Author: Junchao-Mellanox

src/sonic_ax_impl/mibs/ietf/rfc2737.py

@@ -675,7 +675,7 @@ def _update_per_namespace_data(self, pubsub):
 
             db_entry = msg["channel"].split(":")[-1]
             data = msg['data']  # event data
-            if not isinstance(data, bytes):
+            if not isinstance(data, str):
                 continue
 
             # extract interface name