Filters down a list of URL candidates
    :param candidates: list, URL candidates
    :return: set, Repo URLs

def filter_repo_urls(candidates):
    """
    Filters down a list of URL candidates
    :param candidates: list, URL candidates
    :return: set, Repo URLs
    """
    # first, we are going to filter down the URL candidates to be all valid urls
    candidates = set(url for url in [validate_url(_url) for _url in candidates] if url)
    logger.info("Got repo candidates {}".format(candidates))
    repos = set(url for url in [validate_repo_url(_url) for _url in candidates] if url)
    logger.info("Filtered initial candidates down to {}".format(repos))

    return repos