Using Boto3 and Python How Make a Folder Public

I want to make a folder called img that already exists within my private bucket public. I'm using Boto3. I just want to make this folder public not anything else using a script..

This is how I'm currently connecting to the bucket and how far I have got....

ACCESS_KEY_ID = 'xxxxx'
ACCESS_KEY_SECRET = 'xxxx'
bucket_name = 'mybucket'
sourceDir = "../../docs/buildHTML/html/"
destDir = ''

r = boto3.setup_default_session(region_name='eu-west-1')
s3 = boto3.resource('s3', aws_access_key_id=ACCESS_KEY_ID, aws_secret_access_key=ACCESS_KEY_SECRET)
bucket = s3.Bucket(bucket_name)

So I have the bucket and this works. How do I now make the folder img that already exists public?