Download update info and parse it.

def get_online_version ():
    """Download update info and parse it."""
    session = requests.session()
    page = urlopen(UPDATE_URL, session)
    version, url = None, None
    for line in page.text.splitlines():
        if line.startswith(VERSION_TAG):
            version = line.split(':', 1)[1].strip()
        elif line.startswith(URL_TAG):
            url = line.split(':', 1)[1].strip()
            url = url.replace('${version}', version)
    return version, url