Issue #6683: For SMTP logins we now try all authentication methods advertised

Issue #6683: For SMTP logins we now try all authentication methods advertised
by the server. Many servers are buggy and advertise authentication methods they
o not support in reality. This change makes smtplib.auth() work more often in
the real world, where we face misconfigured servers and servers that advertise
methods they don't support due to the madness that is SASL.