fix: use format strings instead of os.path.join for S3 URI in S3Downloader (aws#1520)

Author: laurenyu

src/sagemaker/s3.py

@@ -14,7 +14,6 @@
 from __future__ import print_function, absolute_import
 
 import logging
-import os
 
 from six.moves.urllib.parse import urlparse
 from sagemaker.session import Session
@@ -184,4 +183,4 @@ def list(s3_uri, session=None):
         bucket, key_prefix = parse_s3_url(url=s3_uri)
 
         file_keys = sagemaker_session.list_s3_files(bucket=bucket, key_prefix=key_prefix)
-        return [os.path.join("s3://", bucket, file_key) for file_key in file_keys]
+        return ["s3://{}/{}".format(bucket, file_key) for file_key in file_keys]