We had a few places in the codebase that were still dealing with JSON

payloads using the wrong string type. json.loads() on Python 3

requires a Unicode string, and won't work with a byte string. We were

using the wrong type in some unit tests and when validating DVCS commit

payloads.
This updates the call sites to encode or decode as necessary in order to

provide json.loads() with Unicode strings.

Unit tests pass on all supported versions of Python.