Why am I getting 403 Forbidden whatever I do?

I'm trying to use boto to automate some operations on amazon ec2, but I can't even run the most simple example without getting:

boto.exception.EC2ResponseError: EC2ResponseError: 403 Forbidden

My code is:

import boto.ec2

conn = boto.ec2.connect_to_region("us-west-2",
aws_access_key_id='CENSOREDCENSORED',
aws_secret_access_key='CENSOREDCENSOREDCENSOREDCENSORED')

instances = conn.get_all_instances()

I'm sure that aws_access_key_id and aws_secret_access_key are correct, because I use them in the CLI Tools and I can succesfully run ec2-describe-instances

I'm using boto version: 2.35.1