Refactor version retrieval in setup.py to read from __init__.py

[whyvineet]

Resolves #914

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[mhucka]

@whyvineet could you go ahead and look into signing the contributor's license agreement (CLA) mentioned by the google-cla bot comment earlier on this PR?

I realize that the concept of a CLA may be something unfamiliar. It is a common request by companies to external contributors. It allows contributors to Google open-source projects to retain ownership of their code while granting Google the necessary rights to use those contributions. The CLA covers all Google projects, and it only needs to be signed once.

[whyvineet]

Hey @mhucka, I have already signed the CLA.

[mhucka]

Hey @mhucka, I have already signed the CLA.

Oh, sorry about that! I should have checked first.

Thanks for taking care of that, and for your work.

[mhucka]

@whyvineet Could you go ahead and update this branch? (I could also do it, but it's generally better if contributors do it themselves.)