Returns True if the negative group requirement (AUTH_LDAP_DENY_GROUP)
        is met. Always returns True if AUTH_LDAP_DENY_GROUP is None.

def _check_denied_group(self):
        """
        Returns True if the negative group requirement (AUTH_LDAP_DENY_GROUP)
        is met. Always returns True if AUTH_LDAP_DENY_GROUP is None.
        """
        denied_group_dn = self.settings.DENY_GROUP

        if denied_group_dn is not None:
            is_member = self._get_groups().is_member_of(denied_group_dn)
            if is_member:
                raise self.AuthenticationFailed(
                    "user does not satisfy AUTH_LDAP_DENY_GROUP"
                )

        return True