make HyCons iter return non-iterable cdr

[gilch]

This makes the HyCons __iter__ generator return the last cdr when it's non-iterable. The old behavior was to yield the cdr as the last element and raise a TypeError.

It seems more useful for a dotted list iter to return its cdr than yield it and then raise a TypeError like before. A yield should only be used on the cars, not the cdr. But we still want to be able to access the final cdr. This will make dotted lists easier to work with, since you will be able to use an iterator for them too. Before, you'd have to build one yourself with a while loop or recursion.

I noticed this possibility in #1360, but the __repr__ method still shouldn't use this, because a non-HyCons cdr needs to print differently, even if it's iterable. These are separate issues.

On Python 3.3+, this would just be a return self.cdr statement. I explicitly raised the StopIteration exception for compatibility with Python 2.7, which doesn't allow return in generators at all. On Python3, the returned value is in the .value attr of the exception. On Python2, it's still available in .args, though I suppose we could explicitly set a .value attr on the exception object before raising it.

[gilch]

The failing test is only for enforcing the old behavior. This doesn't seem to have caused any other problems. I'll update it for the new expected behavior.

[Kodiologist]

Is this ready for review?

[gilch]

It looks like the next Python release will break this. It will be an error to do it Python 2's way with an explicit exception. We have to use return. Short of evaling strings conditional on the Python version, I don't see a way to make this work in both versions. We might have to wait until we drop Python2.