Update actions/checkout action to v6 (#825) #5923

Job	Run time
Shellcheck	7s
Dependency	5s
Cache identify	6s
Prepare	11s
Ruff check and force	12s
Check commit	18s
Run pytest using Python 3.13	18s
Run pytest using Python 3.14	20s
Run mypy	13s
Process test coverage	19s
Process test complexity	8s
Build and publish Python 🐍 distributions 📦 to TestPyPI	10s
	2m 27s

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