Convert auto_gen_test_config.py to python 3

This script is run by soong, but without specifying
the python launcher, so it uses the default "python"
in the path, which is python 2.

Convert the script to use python 3.

Bug: 203436762
Test: Building an internal product that uses this
Change-Id: I4496c03b8669c54d080aa45c446dd1f27d91259e