Remove unnecessary use of six.binary_type

[jdufresne]

The bytes type is available on all support Pythons. On Python 2 it is an alias of str (same as six). Reduce unnecessary compatibility shims and by using modern Python idioms. Makes the code more forward compatible with Python 3.

[gsnedders]

I'm mildly amazed this is actually the only place we use it, but seemingly it is! I presume elsewhere we check for text_type and branch if/else on that.