def METHOD_NAME(
        access_token_url, client_id, client_secret_key):
    """Given an API client (id and secret key) that is allowed to make API
    calls, return an access token that can be used to make calls.
    """
    response = requests.post(
        access_token_url,
        headers={
            "Authorization": u"Basic {0}".format(
                base64.b64encode(
                    "{0}:{1}".format(
                        client_id, client_secret_key
                    ).encode()
                ).decode('utf-8')
            ),
        })
    if response.status_code != 200:
        raise AuthorizationError(response.status_code, reponse.text)
    response_json = response.json()
    access_token_expiry_time = time.time() - 2 + response_json["expires_in"]
    return response_json["access_token"], access_token_expiry_time