mypy: simplify configuration #369

Job	Run time
cwltest release test	3m 45s
CI tests via Tox (3, 8, lint)	19s
CI tests via Tox (3, 8, unit)	1m 5s
CI tests via Tox (3, 8, mypy)	43s
CI linters via Tox (lintreadme)	25s
CI tests via Tox (3, 8, bandit)	16s
CI linters via Tox (pydocstyle)	18s
CI tests via Tox (3, 9, lint)	19s
CI tests via Tox (3, 9, unit)	1m 10s
CI tests via Tox (3, 9, mypy)	44s
CI tests via Tox (3, 9, bandit)	17s
CI tests via Tox (3, 10, lint)	19s
CI tests via Tox (3, 10, unit)	1m 6s
CI tests via Tox (3, 10, mypy)	42s
CI tests via Tox (3, 10, bandit)	14s
CI tests via Tox (3, 11, lint)	17s
CI tests via Tox (3, 11, unit)	1m 7s
CI tests via Tox (3, 11, mypy)	34s
CI tests via Tox (3, 11, bandit)	15s
CI tests via Tox (3, 12, lint)	15s
CI tests via Tox (3, 12, unit)	1m 46s
CI tests via Tox (3, 12, mypy)	1m 8s
CI tests via Tox (3, 12, bandit)	20s
	17m 24s

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