Fix #166, making sure we always cast raw tag override values to string

[thibaudcolas]

Description

Fixes #166, following the suggestion from @zerolab. I chose to do this for all Django versions rather than adding a conditional with version check, but happy to revisit this if people think it’s worthwhile.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation Not relevant
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

Suggested CHANGELOG entry:

### Changed

- Fix potential Django 4.0 compatibility issue for components using non-string values in tag overrides ([#166](https://github.com/torchbox/django-pattern-library/issues/166)).