feat: extending python 3 support

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The project runs fine with python 3.6 and tf.1.10, but these changes are not compatible with python2.

[1]: Python 3 changes return values of several basic functions from list to iterator. The main reason for this change is that iterators usually cause better memory consumption than lists.

1: https://portingguide.readthedocs.io/en/latest/iterators.html

[4F2E4A2E]

ping