Move to boto3 and port p-s3

According to Rajiv, we need to move to boto3 for pegasus-s3:

boto is compatible with 2.6 - 3.4 - It may work with > 3.5, but it definitely isn't supported.
https://pypi.org/project/boto/

boto3 is compatible with >= 2.7
https://pypi.org/project/boto3/