Add a package for py-gsutil, version 4.28.

gsutil is a Python application that lets you access Google Cloud Storage and
Amazon S3 from the command line. You can use gsutil to do a wide range of
bucket and object management tasks, including:

* Creating and deleting buckets
* Uploading, downloading, and deleting objects
* Listing buckets and objects
* Moving, copying, and renaming objects
* Editing object and bucket ACLs

(bsiegert)