releasetools: Call Cleanup() in check_target_files_signatures.py.

Also minor cleanups to make it pylint clean.

Test: Run check_target_files_signatures.py with a target-files.zip.
Test: pylint --rcfile=pylintrc check_target_files_signatures.py
Change-Id: Ife3b54c7805c2f2562e87e91ab4b4de355782012