Request the friends list of a given steam ID filtered by role.

        steamID: The user ID
        relationship: Type of friend to request (all, friend)
        format: Return format. None defaults to json. (json, xml, vdf)

def get_friends_list(self, steamID, relationship='all', format=None):
        """Request the friends list of a given steam ID filtered by role.

        steamID: The user ID
        relationship: Type of friend to request (all, friend)
        format: Return format. None defaults to json. (json, xml, vdf)

        """
        parameters = {'steamid' : steamID, 'relationship' : relationship}
        if format is not None:
            parameters['format'] = format
        url = self.create_request_url(self.interface, 'GetFriendsList', 1,
            parameters)
        data = self.retrieve_request(url)
        return self.return_data(data, format=format)