Add Python 3.12 to test matrix and add classifier #458

Job	Run time
Check for news fragments in 'changelog.d/'	7s
Lint (black)	30s
Lint (flake8)	38s
Test Python 3.8, Django 3.2, Redis.py latest	57s
Lint (isort)	36s
Test Python 3.8, Django 4.0, Redis.py latest	1m 1s
Lint (mypy)	38s
Test Python 3.8, Django 4.1, Redis.py latest	51s
Test Python 3.8, Django 4.2, Redis.py latest	1m 1s
Test Python 3.9, Django 3.2, Redis.py latest	49s
Test Python 3.9, Django 4.0, Redis.py latest	1m 0s
Test Python 3.9, Django 4.1, Redis.py latest	52s
Test Python 3.9, Django 4.2, Redis.py latest	1m 2s
Test Python 3.10, Django 3.2, Redis.py latest	56s
Test Python 3.10, Django 4.0, Redis.py latest	50s
Test Python 3.10, Django 4.1, Redis.py latest	52s
Test Python 3.10, Django 4.2, Redis.py latest	1m 2s
Test Python 3.6, Django 3.2, Redis.py latest	4m 38s
Test Python 3.7, Django 3.2, Redis.py latest	4m 28s
Test Python 3.11, Django 4.1, Redis.py latest	59s
Test Python 3.11, Django 4.2, Redis.py latest	57s
Test Python 3.12, Django 4.2, Redis.py latest	43s
Test Python 3.11, Django 4.2, Redis.py master	57s
Test Python 3.11, Django main, Redis.py latest	1m 59s
Test Python 3.11, Django main, Redis.py master	1m 12s
	29m 35s

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