Update to 3.13. From the changelog:

* Drop support for Python 3.5, add support for Python 3.9
* Switch to feedparser 6
* Switch to poetry instead of requirements.txt
* Make the `verbose` flag in the config file actually have an impact,
  and have it default to `info`
* Improve log messages
* Remove documentation of `smtp-ssl-protocol` as this option was
  dropped in 2016
* Stop forging SMTP and sendmail envelope sender (#134)
* Add sendmail_config option
* Log sendmail output
* Support multipart/alternative emails with both HTML and plain text
  parts with option `multipart-html`
* Add inline-links option, allowing links to be sent to the bottom of
  the paragraph
* Add wrap-links option, preventing links from be wrapped over
  multiple lines
* Stop looking in $XDG_DATA_DIRS for the database, and only look in
  $XDG_DATA_HOME
* Warnings about HTTP content-type being unexpected now properly display
* Make the proxy parameter also affect https connections
* Add a --clean argument on the run command to reduce the database size
* Set body element attribute dir=auto in HTML mail
* Store the lock file in XDG_RUNTIME_DIR instead of /tmp


(schmonz)