Log Message:
Update to 3.12. From the changelog:

* Drop support for EOL Python 3.4, add support for Python 3.8
* Add List_ID and List_Post headers to the generated emails
* Add a new `reply-changes` setting
* Improve configurability of text wrapping for the emails
* Use `platform.node()` instead of "dev.null.invalid" in
  "Message-ID" header
* Improve locking support for when multiple rss2email instances are run
  in parallel
* Fix handling of __VERSION__ and __URL__ in user-agent strings
* Fix opmlexport