ci: Prepare release of version 2.1.0

[fsbraun]

Description

This PR updates the classifiers, changelog and removes the outdated direct django-treebeard dependency.

Related resources

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Checklist

• [ ] I have opened this pull request against master
• [ ] I have added or modified the tests when changing logic
• [ ] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [ ] 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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d358846) 100.00% compared to head (b31eae5) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #72 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 9 9 Lines 128 128 Branches 7 7 ========================================= Hits 128 128 ```

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