Fix a wrong return type from Base64Field.value_to_string().
Review Request #12538 — Created Aug. 15, 2022 and submitted
Base64Field.value_to_string()has been returning a byte string, but
this isn't actually compliant with what Django expects. This must return
a Unicode string. It's used for the serialization process, which doesn't
byteson Python 3.
This switches the return type to use the correct string type.
Unit tests pass in Djblets and Review Board.
Succesfully serialized a database to JSON.