Stop relying on 2to3 and use six.py for compatibility instead

[mdboom]

This is a significant amount of work, but will pay off (primarily in shorter build times) in the end.

[pelson]

primarily in shorter build times

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

[mdboom]

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.

[mdboom]

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.

[mdboom]

My thinking here is to always include this in every file to make moving code around easier. The things it has are "core" enough that they should probably be considered "built-ins".

[efiring]

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.

[mdboom]

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.

[WeatherGod]

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.