Generate parser from `cwltool` extensions #981

Job	Run time
cwl-utils release test	6m 22s
CI linters via Tox (lint-readme)	20s
CI tests via Tox (3, 9, lint)	14s
CI linters via Tox (pydocstyle)	12s
CI tests via Tox (3, 9, unit)	3m 6s
CI tests via Tox (3, 9, bandit)	25s
CI tests via Tox (3, 9, mypy)	1m 54s
CI tests via Tox (3, 10, lint)	17s
CI tests via Tox (3, 10, unit)	2m 26s
CI tests via Tox (3, 10, bandit)	23s
CI tests via Tox (3, 10, mypy)	57s
CI tests via Tox (3, 11, lint)	14s
CI tests via Tox (3, 11, unit)	2m 14s
CI tests via Tox (3, 11, bandit)	21s
CI tests via Tox (3, 11, mypy)	57s
CI tests via Tox (3, 12, lint)	17s
CI tests via Tox (3, 12, unit)	2m 16s
CI tests via Tox (3, 12, bandit)	24s
CI tests via Tox (3, 12, mypy)	1m 59s
CI tests via Tox (3, 13, lint)	25s
CI tests via Tox (3, 13, unit)	2m 52s
CI tests via Tox (3, 13, bandit)	35s
CI tests via Tox (3, 13, mypy)	1m 11s
	30m 21s

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