I'm getting an error in boto3 that says:

AttributeError: 'str' object has no attribute 'get'

Look at your ~/.aws/config file for the [default] profile. Specifically, look for a stanza with the key s3. If you see that, delete it. [default] is the stanza used by aws login plugin. It seems sometimes it adds this key and when you log into another AWS account (or your session expires and you re-authenticate) the s3 stanza remains. Once it's there, the AWS CLI doesn't process it correctly and throws the AttributeError.