On boto I used to specify my credentials when connecting to S3 in such a way:

import boto
from boto.s3.connection import Key, S3Connection
S3 = S3Connection( settings.AWS_SERVER_PUBLIC_KEY, settings.AWS_SERVER_SECRET_KEY )

I could then use S3 to perform my operations (in my case deleting an object from a bucket).

With boto3 all the examples I found are such:

import boto3
S3 = boto3.resource( 's3' )
S3.Object( bucket_name, key_name ).delete()

I couldn't specify my credentials and thus all attempts fail with InvalidAccessKeyId error.

How can I specify credentials with boto3?

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')