releasetools: Clean up additional modules for Python 3 compatibility.

It also addresses a few linter warnings (mostly regarding indentation).

Bug: 131631303
Test: TreeHugger
Test: `python -m unittest test_ota_from_target_files`
Test: `python3 -m unittest test_ota_from_target_files`
Test: `python build/make/tools/releasetools/check_ota_package_signature.py
           build/make/target/product/security/testkey.x509.pem
           test-keys-signed-ota.zip`
Test: `python3 build/make/tools/releasetools/check_ota_package_signature.py
           build/make/target/product/security/testkey.x509.pem
           test-keys-signed-ota.zip`
Change-Id: I11a745dfec052081ab0282bb028dc27debf71f57