@pablohoffman also suggested that in private conversation :) I'm not a big fan of this idea, because 

a) there are false positives, and
b) 2to3 is of limited utility, and its purpose is to convert code to 3.x, not to add 3.x support.

About false positives: for example, [here](https://github.com/scrapy/scrapy/pull/435/files#diff-b5bcf8c671c40b8e1227a4aa806f6839R94) touching map was unnecessary, and there could be methods named 'next' that are not related to iteration.

So I'd prefer working testing suite for detecting such issues; we can make 'overall' cleanup first (without caring much if some PR will break it) and then start porting modules one by one by making their tests green.