OK, I got a copy of the Subversion sources and the new SSL library looks
like just what is needed here.  Examining the other protocol modules
that already support SSL, I find these things:

httplib has:
HTTPConnection(host[, port[, strict[, timeout]]])
HTTPSConnection(host[, port[, key_file[, cert_file[, strict[, timeout]]]]])

poplib has:
POP3(host[, port[, timeout]])
POP3_SSL(host[, port[, keyfile[, certfile]]])

imaplib has:
IMAP4([host[, port]])
IMAP4_SSL([host[, port[, keyfile[, certfile]]]])

smtplib has:
SMTP([host[, port[, local_hostname[, timeout]]]])
SMTP_SSL([host[, port[, local_hostname[, keyfile[, certfile[, timeout]]]]]])

Now nntplib as it stands has no SSL-using variant.  The existing factory
method is:
NNTP(host[, port[, user[, password[, readermode[, usenetrc]]]]])

What seems most consistent with the other modules is to define:
NNTP_SSL(host[, port[, keyfile[, certfile[, user[, password[,
readermode[, usenetrc]]]]]]])