textproc/py-mkdocs-rss-plugin: Update to 1.16.0

• Support for Python 3.8 was dropped with this release, adjust USES=python accordingly.

• Remove superfluous MASTER_SITES as well, because it's already set by USE_GITHUB. This was overlooked when adding the port.

Changelog:

https://github.com/Guts/mkdocs-rss-plugin/releases/tag/1.16.0