Create the HTTP client with authentication credentials if required.

def _create_http_client():
    """Create the HTTP client with authentication credentials if required."""
    global _http_client

    defaults = {'user_agent': USER_AGENT}
    auth_username, auth_password = _credentials
    if auth_username and auth_password:
        defaults['auth_username'] = auth_username
        defaults['auth_password'] = auth_password

    _http_client = httpclient.AsyncHTTPClient(
        force_instance=True, defaults=defaults,
        max_clients=_max_clients)