Generate parser from `cwltool` extensions #971

Job	Run time
cwl-utils release test	6m 14s
CI linters via Tox (lint-readme)	24s
CI tests via Tox (3, 9, lint)	16s
CI linters via Tox (pydocstyle)	14s
CI tests via Tox (3, 9, unit)	3m 1s
CI tests via Tox (3, 9, bandit)	31s
CI tests via Tox (3, 9, mypy)	1m 56s
CI tests via Tox (3, 10, lint)	16s
CI tests via Tox (3, 10, unit)	3m 5s
CI tests via Tox (3, 10, bandit)	23s
CI tests via Tox (3, 10, mypy)	1m 2s
CI tests via Tox (3, 11, lint)	18s
CI tests via Tox (3, 11, unit)	2m 44s
CI tests via Tox (3, 11, bandit)	25s
CI tests via Tox (3, 11, mypy)	56s
CI tests via Tox (3, 12, lint)	20s
CI tests via Tox (3, 12, unit)	3m 10s
CI tests via Tox (3, 12, bandit)	23s
CI tests via Tox (3, 12, mypy)	1m 10s
CI tests via Tox (3, 13, lint)	22s
CI tests via Tox (3, 13, unit)	3m 6s
CI tests via Tox (3, 13, bandit)	32s
CI tests via Tox (3, 13, mypy)	1m 9s
	31m 57s

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