How to specify credentials when connecting to boto3 S3?

You can create a session:

import boto3
session = boto3.Session(
    aws_access_key_id=settings.AWS_SERVER_PUBLIC_KEY,
    aws_secret_access_key=settings.AWS_SERVER_SECRET_KEY,
)

Then use that session to get an S3 resource:

s3 = session.resource('s3')

You can get a client with new session directly like below.

 s3_client = boto3.client('s3', 
                      aws_access_key_id=settings.AWS_SERVER_PUBLIC_KEY, 
                      aws_secret_access_key=settings.AWS_SERVER_SECRET_KEY, 
                      region_name=REGION_NAME
                      )