Make the coders package compatible with Python 3

The coders package is affect a lot by the fact that Strings are unicode in Python 3.

The planned approach is to

• Prefix bytestrings with 'b' where appropriate
• Replace uses of 'str' with 'bytes' where appropriate
• Use python-modernize to solve syntax and import errors

The goal of this subtask is not to make the coders package completely compatible with Python 3.