[rfc2737] Fix issue: expect redis pubsub data to be str type instead of bytes type

[Junchao-Mellanox]

- 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

- Description for the changelog

[Junchao-Mellanox]

sonic-net#196