Remove six #1115

Job	Run time
Run tests with different pydantic versions	6s
Run tests with coverage	3m 7s
Run linters (flake8)	1m 44s
Run tests (3.8)	1m 56s
Run linters (pydocstyle)	1m 46s
Run tests (3.9)	2m 1s
Run tests on legacy versions (3.7)	2m 8s
Run linters (mypy)	1m 54s
Run tests (3.10)	2m 4s
Run linters (pylint)	1m 53s
Run tests (3.11)	2m 7s
Run tests (3.12)	2m 2s
Run tests (3.13)	2m 10s
	24m 58s

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