TensorFlow 2.0 and Python 3 compatibility.

[chrisying]

Incorporated changes from:

• small changes to make the code compatible with TF2 #25
• Python 3 Compatibility Support #16

Tested via:

• Colab notebook (running Python 3.6.9)
• Ran all unit tests in local environment with virtualenv (Python 3.8.5)

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We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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