py3: catch StopIteration from next() in generatorset
IIUC, letting the StopIteration through would not cause any bugs, but
not doing it makes the test-py3-commands.t pass.
I have also diligently gone through all uses of next() in our code
base. They either:
* are not called from a generator
* pass a default value to next()
* catch StopException
* work on infinite iterators
* request a fixed number of items that matches the generated number
* are about batching in wireproto which I didn't quite follow
I'd appreciate if Augie or someone else could take a look at the
wireproto batching and convince themselves that the next(batchable)
calls there will not raise a StopIteration.
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