Migrate S3ObjectStore from python3-boto to python3-boto3.
BREAKING CHANGE: Some of the methods did not capture all the underlying
boto expeceptions, and exceptions are different between boto and boto3.
Thus, any callsite handling boto exceptions may have to adjust them.
BREAKING CHANGE: S3ObjectStore.bucket did expose a boto class which is
different in boto3. `bucket` has been removed to not couple
simplestreams' API with boto{,3}'s API. S3ObjectStore.bucketname is
still exposed, so callsites can refer to it however they want.
Add the new --skip-aws to be able to don't create the AWS
streams. This is optional and the current default won't change but
it's required because stream generation for AWS will move to a
different system soon.