The main Pyzor server seems to have been down for a couple months now (no wonder my spam load was so high!). Even if you do ‘pyzor discover’ the server it returns is the one that doesn’t respond.
To restore pyzor functionality, change your pyzor/servers file to:
and restart any processes, e.g. spamd or MailScanner which may have the old value in memory. Watch out for any ‘pyzor discover’ cron jobs you might have around.