List all users registered on this Pagure instance.
        :param pattern: filters the starting letters of the return value
        :return:

def list_users(self, pattern=None):
        """
        List all users registered on this Pagure instance.
        :param pattern: filters the starting letters of the return value
        :return:
        """
        request_url = "{}/api/0/users".format(self.instance)
        params = None
        if pattern:
            params = {'pattern': pattern}
        return_value = self._call_api(request_url, params=params)
        return return_value['users']