References:
        https://developer.github.com/v3/issues/labels/#add-labels-to-an-issue

def add_labels_to_pr(repo: GithubRepository,
                     pull_id: int,
                     *labels: str,
                     override_token: str = None) -> None:
    """
    References:
        https://developer.github.com/v3/issues/labels/#add-labels-to-an-issue
    """
    url = ("https://api.github.com/repos/{}/{}/issues/{}/labels"
           "?access_token={}".format(repo.organization,
                                     repo.name,
                                     pull_id,
                                     override_token or repo.access_token))
    response = requests.post(url, json=list(labels))

    if response.status_code != 200:
        raise RuntimeError(
            'Add labels failed. Code: {}. Content: {}.'.format(
                response.status_code, response.content))