boto3 updates

[e-maud]

• update get_bucket to boto3 and get read of boto
• change signature of get_s3_versions so that the first param is an instantiated bucket already.
• update smart_open library
• update boto3 library
• detect number of workers automatically for s3_iter_bucket function (default is 16)

[e-maud]

The last release of smart_open library, fron which we get the s3_iter_bucket still works with boto.

The last version of the files on github of this library use boto3.

=> we wait until the release of smart open with boto3, and live with the 2 boto for now.