disable verification of ssl certs via CA if fingerpint is set

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

offlineimap/imapserver.py

@@ -95,6 +95,9 @@ def __init__(self, repos):
             self.__verifycert = None  # Disable cert verification.
             # This way of working sucks hard...
         self.fingerprint = repos.get_ssl_fingerprint()
+        if self.fingerprint is not None:
+            self.__verifycert = None  # if we have a cert fingerprint
+            # we do not require verfication via CA
         self.tlslevel = repos.gettlslevel()
         self.sslversion = repos.getsslversion()
         self.starttls = repos.getstarttls()