Performs the import only once.

def import_it(cls):
        """
        Performs the import only once.
        """
        if not cls in cls._FEATURES:
            try:
                cls._FEATURES[cls] = cls._import_it()
            except ImportError:
                raise cls.Error(cls._import_error_message(), cls.Error.UNSATISFIED_IMPORT_REQ)
        return cls._FEATURES[cls]