The dependency on six was problematic, as it was getting imported

during the packaging process by way of kgb/__init__.py importing the

module that imports six, and this was imported by setup.py. That meant

that packaging would fail if six was not already installed. Since we

need so very little from it (iteritems and text_type), we just

define our own wrapper types.
While testing, I noticed my prior change also accidentally broke tests

on Python 3. Those are fixed with this change by importing our

text_type and plugging in its type name in some error messages instead

of assuming they're unicode.

Unit tests pass on Python 2 and 3.
I was able to build a package from a fresh virtualenv.