Return the :class:`~eth.db.account.BaseAccountDB` class that the
        state class uses.

def get_account_db_class(cls) -> Type[BaseAccountDB]:
        """
        Return the :class:`~eth.db.account.BaseAccountDB` class that the
        state class uses.
        """
        if cls.account_db_class is None:
            raise AttributeError("No account_db_class set for {0}".format(cls.__name__))
        return cls.account_db_class