In an effort to get a sense of how useful this would actually be, I did a code search for `.isoformat()` on github. I saw a few doctests that will break (if they're even being run) if we make this change, but I also found that the *vast* majority of uses of `isocalendar` seem to be people pulling out a single component of it, like:  `return datetime.datetime.now().isocalendar()[1]`.

That is not the kind of usage pattern I was envisioning when I said that this was a marginal improvement, a *lot* of this code could be made considerably more readable with named fields. If indeed the performance is similar or the same and this won't impact consumers of the pure python version of the module unduly (I checked in #pypy and they think that it shouldn't be more than a minor irritation if anything), then I am changing my -1 to a +1.