Update cms_plugins.py - isort/flake8 check

Description

Make the plugin work with Django CMS v4. Also works with v3 by using the cms.api.

[x] I have opened this pull request against master
[ ] I have added or modified the tests when changing logic
[x] I have followed the conventional commits guidelines to add meaningful information into the changelog
[x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.