Despite the name, django-rss-plugin is a small wrapper around feedparser, which presents an RSS feed as a plugin for django-cms. This plugin is currently not updated for django 3.x (c.f. #52). The current maintainer is a finnish software (https://github.com/andersinno/), that regularly works on django software but hasn't touched this particular repo in over a year (https://github.com/andersinno/django-rss-plugin/).
I'll try to move this along to django 3.x and see if it breaks but we may want to switch to something like djangocms-blog in the long run, which is officially maintained by https://github.com/nephila/, which is a gold-member of the django-cms association. I proposed this in the past, which didn't work out, so this is mostly just a note.