CONTRIBUTING.md: Add info on running linters and simplify mypy invocation #1242

Job	Run time
Run CI / Build / Build	55s
Run CI / Generate docs / Generate docs	53s
Run CI / Run unit tests / Run unit tests (ubuntu-latest, 3.8)	51s
Run CI / Run unit tests / Run unit tests (ubuntu-latest, 3.9)	43s
Run CI / Run unit tests / Run unit tests (ubuntu-latest, 3.10)	53s
Run CI / Run unit tests / Run unit tests (ubuntu-latest, 3.11)	41s
Run CI / Run unit tests / Run unit tests (ubuntu-latest, 3.12)	1m 0s
Run CI / Run unit tests / Run unit tests (windows-latest, 3.8)	1m 26s
Run CI / Run unit tests / Run unit tests (windows-latest, 3.9)	1m 36s
Run CI / Run unit tests / Run unit tests (windows-latest, 3.10)	1m 24s
Run CI / Run unit tests / Run unit tests (windows-latest, 3.11)	1m 45s
Run CI / Run unit tests / Run unit tests (windows-latest, 3.12)	1m 20s
Run CI / Run system tests / Run system tests (win-10-py32)	37m 14s
Run CI / Run system tests / Run system tests (win-10-py64)	37m 10s
Run CI / Report test results / Report test results	21s
	1h 28m 12s

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