Enable unsigned request when listing S3 bucket

[ErinWeisbart]

Would like for pe2loaddata to be able to make csvs from datasets in cellpainting-gallery.
To list data in the cellpainting-gallery you need to be able to make an unsigned request. Using the CLI it's a --no-sign-request flag. Using boto3 it's less clean, but I've gotten started on it in f9566e0

[ErinWeisbart]

Instead of putting the no-sign in the client session it looks like it also works to do it this way in the listing itself:

s3 = boto3.client('s3')
plate_list = s3.list_objects(
    Bucket=bucket, Prefix=prefix, Delimiter='/', RequestPayer='no-sign-request')