Update django-ckeditor to 5.9.0

[pyup-bot]

This PR updates django-ckeditor from 5.7.1 to 5.9.0.

Changelog

5.9.0

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. Django 3.0 support
. Python 3.8 support
. Replace `staticfiles` templatetags library usage with `static`
. Add a templates validation step to the tests
. Internationalize ckeditor_upload `browse.html` template.
. Add ckeditor_upload features and custom configuration example to
`ckeditor_demo`

5.8.0

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Links

• PyPI: https://pypi.org/project/django-ckeditor
• Changelog: https://pyup.io/changelogs/django-ckeditor/
• Repo: https://github.com/django-ckeditor/django-ckeditor