gh-113971: Make `zipfile.ZipInfo._compresslevel` public as `.compress_level` (#113969)

Make zipfile.ZipInfo.compress_level public.

A property is used to retain the behavior of the ._compresslevel.

People constructing zipfile.ZipInfo instances to pass into existing APIs to control per-file compression levels already treat this as public, there was never a reason for it not to be.

I used the more modern name compress_level instead of compresslevel as the keyword argument on other ZipFile APIs is called to be consistent with compress_type and a general long term preference of not runningwordstogether without a separator in names.