textproc/py-mkdocs-rss-plugin: Update to 1.17.2
- Switch to the PEP517 build framework.
- Also adjust USES=python accordingly, because Python 3.8 is no longer present in the ports tree after ca15850a2b13 while I'm here.
Changelog:
https://github.com/Guts/mkdocs-rss-plugin/releases/tag/1.17.2