Abstract away some types
String and integer types have changed in python 3. In 2.x, the str type is a

string of bytes, and the unicode type is a string of text. In 3.x, the

bytes type has replaced the old str, and str has replaced unicode. The

six library has abstractions for these that we can use while we're trying to

support both.
Additionally, 3.x has merged the int and long types, so there's a similar

abstraction for that.
There's still a lot of more complicated stuff to do with how we handle strings,

because of the changes to string literals. The stuff where we handle encoding

for diffs will require some special attention, I think.

Ran unit tests.