keeping the conversation in one place, the code review comment was: "I've prefixed this with an underscore because it's not exposed when reading back. ZipInfo has another "private" attribute, _raw_time"

compresslevel is documented on the public APIs where it appears, I do not believe it needs to be a public attribute within ZipInfo given the above as that is an implementation detail.  If we ever find a good reason to make it public in the future we can add a property.