textproc/py-mkdocs-rss-plugin: Update to 1.17.1

* Belatedly remove old cruft related to older Python versions as the
  port requires Python 3.9 at a minimum for runtime.

Changelog since 1.16.0:

https://github.com/Guts/mkdocs-rss-plugin/blob/1.17.1/CHANGELOG.md