None of the up-voted answers work for me. Finally I pass the credentials inside the python script, using the client API.
import boto3
client = boto3.client(
's3',
aws_access_key_id=ACCESS_KEY,
aws_secret_access_key=SECRET_KEY,
aws_session_token=SESSION_TOKEN)
Please notice that the aws_session_token argument is optional. Not recommended for public work, but make life easier for simple trial.