Update mixer requirement from ==6.0.* to ==6.1.* (#358)
Update mixer requirement from ==6.0.* to ==6.1.* (#358)

Updates the requirements on [mixer](https://github.com/klen/mixer) to permit the latest version.
- [Release notes](https://github.com/klen/mixer/releases)
- [Changelog](https://github.com/klen/mixer/blob/develop/Changelog)
- [Commits](https://github.com/klen/mixer/compare/6.0.0...6.1.3)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
sanoma/django-arctic,sanoma/django-arctic,sanoma/django-arctic