feat: Add read-only mixin for a custom Django admin class

[doctoryes]

Description:

The mixin disables CRUD operations on the admin's model. This type of mixin is currently used in an edx-platform Django proxy model, used for invoking actions on certain course keys - but without modifying the proxied model.

Bump version to 4.1.0 for release.

JIRA:

https://openedx.atlassian.net/browse/TNL-8080

Reviewers:

• [ ] tag reviewer

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

Author concerns:

List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.