You are not using the session you created to download the file, you're using s3 client you created. If you want to use the client you need to specify credentials.
your_bucket.download_file('k.png', '/Users/username/Desktop/k.png')
or
s3 = boto3.client('s3', aws_access_key_id=... , aws_secret_access_key=...)
s3.download_file('your_bucket','k.png','/Users/username/Desktop/k.png')
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