Split an S3-prefixed URL into bucket and path.

def bucket_and_path(self, url):
        """Split an S3-prefixed URL into bucket and path."""
        url = compat.as_str_any(url)
        if url.startswith("s3://"):
            url = url[len("s3://"):]
        idx = url.index("/")
        bucket = url[:idx]
        path = url[(idx + 1):]
        return bucket, path