def get_policy_version(client, arn: str) -> str:
    """Get the default version of the policy with this ARN.

    This is opposite of the AWS API name where `get_policy` gives you information about a policy's
    current version but not the policy's contents.
    """

    policy_info = client.get_policy(PolicyArn=arn)
    return policy_info["Policy"]["DefaultVersionId"]