releasetools: Make merge_target_files and its test Python 3 compatible.

And a few minor clean-ups to the styling.

Bug: 131631303
Test: python -m unittest test_merge_target_files
Test: python3 -m unittest test_merge_target_files
Test: Use `python merge_target_files` to merge two target_files zips.
Test: Use `python3 merge_target_files` to merge two target_files zips.
Change-Id: I8502dfb243408f658d022e8d5e5fbb60066e4ff0