Fix tflite_convert.py issue in Python 3

This fix tries to address the issue raised in 20276 where
in python 3, the following error were thrown with tflite_convert.py:
```
TypeError: object of type 'zip' has no len().
```
This fix converts zip output to list to fix the issue in python 3.

This fix fixes 20276.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>