tests: add ujson python3 dependency

Signed-off-by: David Aguilar <davvid@gmail.com>
handlers: handle byte strings when using base64

Add helper methods to paper over the byte strings used by the
base64 module on python3.  This prevents `bytes` objects from
entering the json stream.

Signed-off-by: David Aguilar <davvid@gmail.com>