Over time, we've accumulated a number of unused imports and variables in

the codebase that were never cleaned up, and along with this we had

other warnings for unused (but wanted) imports, some mixed string types,

bad/missing variables, and code using unicode that should have been

six.text_type.
This change simply cleans up all that, helping keep the code

maintainable and making it just a bit easier to start porting things

over to Python 3.

All unit tests pass.