Django 4.0 compatibility

[blag]

This PR adds Django 4 compatibility.

Changes proposed in this PR:

• Add support for Django 4+
• Keep compatibility for Django < 4.0

Tips for an ideal PR

• You have read our Code of Conduct: https://github.com/pinax/.github/blob/master/CODE_OF_CONDUCT.md
• You have read our Contributing info: https://github.com/pinax/.github/blob/master/CONTRIBUTING.md
• The PR is atomic
• Unit tests have been updated/added, if needed - N/A
• App version number has been updated in setup.py (Pinax uses SemVer)
• Change Log has been updated - N/A
• You have added your name to the AUTHORS file