How to use the africastalking.Payment.PaymentService function in africastalking

def __init__(self, username, api_key):
        super(PaymentService, self).__init__(username, api_key)

def initialize(username, api_key):

    if username is None or api_key is None:
        raise RuntimeError('Invalid username and/or api_key')

    globals()['SMS'] = SMSService(username, api_key)
    globals()['Airtime'] = AirtimeService(username, api_key)
    globals()['Payment'] = PaymentService(username, api_key)
    globals()['Voice'] = VoiceService(username, api_key)
    globals()['Application'] = ApplicationService(username, api_key)
    globals()['Token'] = TokenService(username, api_key)
    # globals()['USSD'] = USSDService(username, api_key)