Updated tf-records tutorial for compatibility with both py2 and py3.

[rachellim]

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CLAs look good, thanks!

[MarkDaoust]

Thanks for pointing this out!
I've already got a PR updating most of this doc. That maybe just needs a Merge: #188

I've updated it to be py3 by default and to run successfully in both. This way we don't get merge conflicts (merge with ipynb-json will be bad).

So I'm closing this, and I'll ping the review on 188.