primarily in shorter build times

And the fact that we would have a single code base which users of Python3 can actually contribute against...

Well, I'm not sure that using six vs. 2to3 requires any less knowledge of the differences between Python 2 and 3 to be effective at it, unfortunately. As I work through this, it certainly doesn't result in "cleaner" code.

Ok -- here it is. This is going to make some merges harder, unfortunately. Also, we should probably get #2281 (which is against 1.3.x) finished and merged with this before finalizing this. Some of these changes may be surprising, and I'll try to notate them here in this PR.

Mike,
Your explanations are very helpful. I think it would be a good idea to include them in the code as comments rather than leaving them only as comments in the PR. The things you are explaining are the sorts of things a person might trip over when working with the code later, so they are exactly the sort of thing for which in-code comments are needed. This will also be helpful to others, including myself, when trying to migrate other code from 2-only to single codebase 2-or-3.
Putting these examples and explanations in a file of some sort that becomes part of the documentation (not necessarily sphinxified--could be just a plain text file) would also be helpful.

For some of these, there are many instances of them, and I wouldn't want to comment them in each place. But perhaps a "writing portable code" document in the developer docs is in order.

And a reference to that document in the relevant source files for the most
tricky changes might also help bring that to the attention of any prying
eyes.