removed `sat_tmp.twisted` patches + update minimum Twisted version: - `sat_tmp` patches are not needed anymore and cause trouble with latest Twisted (21.02) - update minimum to `20.3.0` which is the first one to support Python 3.8

create table callbacks (
    service text not null,
    node text not null,
    uri text not null,
    PRIMARY KEY (service, node, uri)
);