Up to a4863b2 offlineimap did not include a default CA bundle. And
folks who set cert_fingerprint (because they might connect to a
host using a self-signed cert or an onion service without the onion
address in the SANs) were able to validate their certificates.
Since a4863b2 you always have a `sslcacertfile` configured (since
it always falls back to the os one) and thus the old way didn't
work anymore.
If a use defines a `cert_fingerprint` there is not much use to
validate the cert through the CA chain, since the fingerprint
is the stronges verification you can get. Therefor we can disable
verfication when `cert_fingerprint` is set.
This enables users to fetch emails again from onion services or
hosts using self-signed certifcates, but doesn't question nor
change any other behavior.
Fixes#41
871b1175f1