support plugins for alternative FsAccess #456

Job	Run time
CI linters via Tox (lintreadme)	21s
cwltest release test	1m 5s
CI linters via Tox (pydocstyle)	16s
CI tests via Tox (3, 8, lint)	24s
CI tests via Tox (3, 8, unit)	1m 15s
CI tests via Tox (3, 8, mypy)	47s
CI tests via Tox (3, 8, bandit)	12s
CI tests via Tox (3, 9, lint)	14s
CI tests via Tox (3, 9, unit)	1m 15s
CI tests via Tox (3, 9, mypy)	46s
CI tests via Tox (3, 9, bandit)	18s
CI tests via Tox (3, 10, lint)	16s
CI tests via Tox (3, 10, unit)	1m 15s
CI tests via Tox (3, 10, mypy)	32s
CI tests via Tox (3, 10, bandit)	12s
CI tests via Tox (3, 11, lint)	12s
CI tests via Tox (3, 11, unit)	1m 27s
CI tests via Tox (3, 11, mypy)	30s
CI tests via Tox (3, 11, bandit)	15s
CI tests via Tox (3, 12, lint)	14s
CI tests via Tox (3, 12, unit)	1m 28s
CI tests via Tox (3, 12, mypy)	47s
CI tests via Tox (3, 12, bandit)	12s
CI tests via Tox (3, 13, lint)	25s
CI tests via Tox (3, 13, unit)	1m 30s
CI tests via Tox (3, 13, mypy)	58s
CI tests via Tox (3, 13, bandit)	20s
	17m 26s

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