[gyb] Use lambda function to work around PEP 3114
[gyb] Use lambda function to work around PEP 3114

[PEP 3114](https://www.python.org/dev/peps/pep-3114/) renamed
`iterator.next()` (Python 2) to `iterator.__next__()` (Python 3). The
recommended solution to make the code work in both Python 2 and 3 is to
call the global `next` function.

To use this recommended global `next` function this patch uses a lambda
function to supply `tokenize.generate_tokens` a callable function as it was previously.

This should be functionally equivalent to the old code with the added
benefit of working on both Python 2 and 3.

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