[issue2551351] mailer.py tighten up ssl connection

New submission from John Rouillard:

When we have python 3.6 or newer we can set options in mailer.py to validate the remote 
certificate name/subject_alt_name and load default certs from the hosts certificate store.

We need tests to verify this works.

Consider a test server like:

  mailpit (https://mailpit.axllent.org/docs/configuration/)
  smtp4dev (https://github.com/rnwood/smtp4dev)
  greenmail (https://greenmail-mail-test.github.io/greenmail/#features)

that can do starttls (initial connection tcp then upgrade to TLS)
or TLS/SSL (all connections over TLS). (Note we can't do TLS/SSL
we only support starttls.)

----------
components: Mail interface
messages: 8047
nosy: rouilj
severity: normal
status: new
title: mailer.py tighten up ssl connection
type: security

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<https://issues.roundup-tracker.org/issue2551351>
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