fix: allow download_folder to download file even if bucket is more restricted

[laurenyu]

Issue #, if available:
#1283

Description of changes:
S3 doesn't provide a way of definitively determining if a prefix is a folder, so originally we used ListObjects to try and gauge that. However, ListObjects requires extra permissions on the bucket, and so download a public file from a private bucket fails. In this PR, I've changed the logic to try and download the file first, and then if that incurs a 404 because it's a folder (no idea why that's how S3 chooses to implement this...), then the code will proceed with as if the prefix points to a folder.

Side note - I noticed that pylint was requiring me to put botocore in the wrong order for the imports. I'll fix that in a separate PR.

Testing done:
tried the example S3 path from #1283:

>>> import sagemaker
>>> from sagemaker import utils
>>> utils.download_folder('lausen-public', 'bert_sst.tar.gz', '/tmp', sagemaker.session.Session())

$ ls /tmp/bert_sst.tar.gz
/tmp/bert_sst.tar.gz

(also ran tox tests/unit)

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[nadiaya]

Suggestion: Could moving it into a function and calling it from inside the except when object is not a file make it better?

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