MySQL JSON field type with string should decode in python3

Table:

`CREATE TABLE test (id int, value json);`

With data:

id | value
---- | ----
1 | {"string1":"string2"}

But event's values in python3 is :

`{"id":1,"value":{b"string1":b"string2"}`

And when use `json.dumps` will raise `TypeError: keys must be a string`

The issue names no source file or test; start by reproducing the MySQL JSON column case through the replication event path in this repository. Trace how the JSON value is decoded in Python 3, then verify that string keys and values remain JSON-compatible so json.dumps succeeds.

Written by the indexing model from the issue text.