add back in Python 3.7 for one last release #1748

Job	Run time
CWL conformance (v1.0, docker)	3m 53s
CWL conformance (v1.0, singularity)	3m 25s
CWL conformance (v1.0, podman)	3m 24s
CWL conformance (v1.1, docker)	6m 0s
CWL conformance (v1.1, singularity)	8m 30s
CWL conformance (v1.1, podman)	5m 8s
CWL conformance (v1.2, docker)	6m 52s
CWL conformance (v1.2, singularity)	8m 16s
CWL conformance (v1.2, podman)	6m 20s
CWL conformance (v1.2, docker, --fast-parser)	5m 51s
build_test_container	12m 50s
No leftovers	6m 31s
Test on macos-latest	6m 57s
cwltool release test	13m 50s
Tox (3, 7, unit)	9m 26s
Tox (3, 8, lint)	34s
Tox (3, 8, unit)	9m 16s
Tox (3, 8, bandit)	1m 2s
Tox (3, 8, mypy)	10m 50s
Tox (3, 9, lint)	35s
Tox (3, 9, unit)	10m 12s
Tox (3, 9, bandit)	31s
Tox (3, 9, mypy)	14m 6s
Tox (3, 10, lint)	42s
Tox (3, 10, unit)	12m 11s
Tox (3, 10, bandit)	33s
Tox (3, 10, mypy)	13m 30s
Tox (3, 11, lint)	53s
Tox (3, 11, unit)	9m 48s
Tox (3, 11, bandit)	43s
Tox (3, 11, mypy)	11m 1s
Tox (3, 12, lint)	55s
Tox (3, 12, unit)	10m 34s
Tox (3, 12, bandit)	1m 11s
Tox (3, 12, mypy)	1m 50s
Linters (lintreadme)	49s
Linters (shellcheck)	30s
Linters (pydocstyle)	47s
	3h 40m 16s

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