We've tried to be future-proof with our string types, using Unicode

strings for most things and byte strings where we thought we needed

them. These weren't always correct, though. There are strings that

needed to be the native string type for the Python version in some

cases, and other places where we had to go back to literal byte strings

or to encode to those.
This change takes care of most of the simpler cases. There are still

other places in the codebase where the wrong string type is being used,

but they're part of bigger changes that will be addressed separately.

Unit tests pass on Python 2.7, 3.4, 3.5, and 3.6 (when used with other

upcoming changes).