Update django-markdownx to 4.0.0

[pyup-bot]

This PR updates django-markdownx from 2.0.28 to 4.0.0.

Changelog

4.0.0

Added:

- Support for Django 4 (retains backward compatibility with Django 3)
- Support for Python 3.9 and 3.10

4.0.0beta.1

Added:
- Support for Django 4 (retains backward compatibility with Django 2 and 3)
- Support for Python 3.9 and 3.10

Changed:
- Minor correction in German translation

Removed:
- Built documentation assets, which are now built via the CI.

3.0.1

Fixed:
- django's internal db field type

3.0.0

Added:
- support for Django 2 and 3
- support for python 3.6, 3.7 and 3.8

Removed:
- support of Django < 2
- python 2.7 code

Fixed:
- minor compatibility issues

Links

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

[pyup-bot]

Closing this in favor of #543