Add Python 3.12 to test matrix and add classifier #463

Job	Run time
Lint (black)	35s
Lint (flake8)	33s
Test Python 3.8, Django 3.2, Redis.py latest	4m 46s
Check for news fragments in 'changelog.d/'	9s
Lint (isort)	35s
Test Python 3.8, Django 4.0, Redis.py latest	4m 49s
Lint (mypy)	34s
Test Python 3.8, Django 4.1, Redis.py latest	4m 50s
Test Python 3.8, Django 4.2, Redis.py latest	4m 50s
Test Python 3.9, Django 3.2, Redis.py latest	4m 36s
Test Python 3.9, Django 4.0, Redis.py latest	4m 42s
Test Python 3.9, Django 4.1, Redis.py latest	4m 38s
Test Python 3.9, Django 4.2, Redis.py latest	4m 35s
Test Python 3.10, Django 3.2, Redis.py latest	5m 0s
Test Python 3.10, Django 4.0, Redis.py latest	4m 40s
Test Python 3.10, Django 4.1, Redis.py latest	4m 40s
Test Python 3.10, Django 4.2, Redis.py latest	4m 36s
Test Python 3.6, Django 3.2, Redis.py latest	4m 46s
Test Python 3.7, Django 3.2, Redis.py latest	4m 54s
Test Python 3.11, Django 4.1, Redis.py latest	4m 52s
Test Python 3.11, Django 4.2, Redis.py latest	4m 50s
Test Python 3.12, Django 4.2, Redis.py latest	1m 5s
Test Python 3.11, Django 4.2, Redis.py master	4m 49s
Test Python 3.11, Django main, Redis.py latest	4m 58s
Test Python 3.11, Django main, Redis.py master	4m 47s
	1h 34m 9s

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