cpu_count can be None, so fallback to 1 #2357

Job	Run time
Linters (lintreadme)	36s
No leftovers	3m 14s
Linters (shellcheck)	14s
CWL conformance (v1.0, docker)	6m 2s
Linters (pydocstyle)	21s
CWL conformance (v1.0, singularity)	6m 2s
CWL conformance (v1.0, podman)	6m 14s
build_test_container	9m 17s
CWL conformance (v1.1, docker)	6m 51s
Test on macos-latest	3m 56s
CWL conformance (v1.1, singularity)	8m 1s
cwltool release test	12m 18s
CWL conformance (v1.1, podman)	7m 15s
CWL conformance (v1.2, docker)	8m 44s
CWL conformance (v1.2, singularity)	9m 17s
Tox (3, 9, lint)	26s
CWL conformance (v1.2, podman)	9m 16s
Tox (3, 9, unit)	4m 46s
CWL conformance (v1.2, docker, --fast-parser)	8m 52s
Tox (3, 9, bandit)	24s
CWL conformance (v1.3.0-dev1, --relax-path-checks)	5m 29s
Tox (3, 9, mypy)	6m 21s
Tox (3, 10, lint)	30s
Tox (3, 10, unit)	4m 28s
Tox (3, 10, bandit)	23s
Tox (3, 10, mypy)	6m 13s
Tox (3, 11, lint)	24s
Tox (3, 11, unit)	4m 33s
Tox (3, 11, bandit)	29s
Tox (3, 11, mypy)	6m 6s
Tox (3, 12, lint)	31s
Tox (3, 12, unit)	6m 38s
Tox (3, 12, bandit)	25s
Tox (3, 12, mypy)	6m 24s
Tox (3, 13, lint)	34s
Tox (3, 13, unit)	6m 22s
Tox (3, 13, bandit)	37s
Tox (3, 13, mypy)	50s
	2h 49m 23s

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