GCS is adding a storage class called "archive" to the existing options for object storage (coldline, standard, etc). Support for this should be added to this client library.

This should be straightforward as ARCHIVE should just mirror existing valid storage classes. I see the following places where changes are required in order to accomplish this:

• https://github.com/googleapis/google-cloud-python/blob/master/storage/google/cloud/storage/constants.py (add new constant)
• https://github.com/googleapis/google-cloud-python/blob/master/storage/google/cloud/storage/bucket.py (add import, update STORAGE_CLASSES, update documentation).
• https://github.com/googleapis/google-cloud-python/blob/master/storage/google/cloud/storage/blob.py (same as above)
• Incorporate ARCHIVE into unit tests as appropriate.

This has already been implemented in the C++ client library; reference commit here