Bug with imap.gmail.com and getmail TLS validation

Hi,

If using imap.gmail.com with the ca_certs option configured to point
at a valid set of CA root certificates then the certificate validation
fails with the following error:

gmail: operation error (socket sslerror during connect ([SSL:
CERTIFICATE_VERIFY_FAILED] certificate verify
failed (_ssl.c:726)))

If you try the same connection using openssl s_client then it
successfully connects and validates.

The reason appears to be an issue with TLSv1.3. If you set ssl_version
= tlsv1_2 in the getmail config then it works as expected.

Googling for a similar issue has shown me this thread:
https://github.com/OfflineIMAP/offlineimap/issues/573 which suggests
that it's something to do with imaplib2 not supporting TLSv1.3 + SNI.
I guess getmail uses the same lib?

It looks like the upstream imaplib2 is completely abandoned. So I
don't know what the best solution is going to be.

Thanks, Matt.