Bump actions/setup-python from 2 to 4 #529

Job	Run time
Check for news fragments in 'changelog.d/'	7s
Lint (black)	53s
Test Python 3.8, Django 4.1, Redis.py latest	4m 32s
Lint (ruff)	22s
Test Python 3.8, Django 4.2, Redis.py latest	4m 34s
Lint (mypy)	29s
Test Python 3.9, Django 4.1, Redis.py latest	5m 1s
Test Python 3.9, Django 4.2, Redis.py latest	4m 32s
Test Python 3.10, Django 4.1, Redis.py latest	4m 41s
Test Python 3.10, Django 4.2, Redis.py latest	4m 29s
Test Python 3.11, Django 4.1, Redis.py latest	4m 56s
Test Python 3.11, Django 4.2, Redis.py latest	4m 34s
Test Python 3.11, Django 4.2, Redis.py master	4m 30s
Test Python 3.11, Django main, Redis.py latest	4m 36s
Test Python 3.11, Django main, Redis.py master	4m 41s
	52m 57s

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