Add support for djangoCMS 4.x

[filipweidemann]

Description

This PR adds support for the new djangoCMS 4.x.
It closes #221 and uses the outlined approach discussed there.

I also refactored the test setup and added the Mixin/Fixture of djangocms-text-ckeditor4 to support both djangoCMS 3 & 4.

All tests are green locally, I'd be happy to get this merged if everything is looking good to you @fsbraun :)

Related resources

• https://github.com/django-cms/djangocms-text-ckeditor/blob/master/tests/fixtures.py

Checklist

• [ x ] I have opened this pull request against master
• [ x ] 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.

[fsbraun]

This PR supersedes #201

[filipweidemann]

Oh man I knew it, I'll quickly sort the test files as well...

[filipweidemann]

@fsbraun I isorted the tests/ directory as well, sorry for the miss there.

[fsbraun]

@filipweidemann Thank you so much for this nice PR!

[filipweidemann]

Ofc, thanks for the merge as well :)

[fsbraun]

@filipweidemann Released as version 4.0 🎉

[filipweidemann]

@fsbraun lovely, thank you so much 🙏