add Django 4.0 (currently in Alpha 1) to test matrix

[beniwohli]

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[basepi]

🎉

[ghost]

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• Start Time: 2021-10-04T09:50:31.459+0000

• Duration: 28 min 48 sec

• Commit: 3d8bd4c

Test stats 🧪

Test	Results
Failed	0
Passed	9951
Skipped	8977
Total	18928

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