Bump sphinx-rtd-theme from 2.0.0 to 3.0.0 #427

Job	Run time
cwltest release test	3m 39s
CI tests via Tox (3, 8, lint)	13s
CI linters via Tox (lintreadme)	19s
CI tests via Tox (3, 8, unit)	1m 8s
CI linters via Tox (pydocstyle)	13s
CI tests via Tox (3, 8, mypy)	38s
CI tests via Tox (3, 8, bandit)	12s
CI tests via Tox (3, 9, lint)	20s
CI tests via Tox (3, 9, unit)	1m 7s
CI tests via Tox (3, 9, mypy)	32s
CI tests via Tox (3, 9, bandit)	13s
CI tests via Tox (3, 10, lint)	13s
CI tests via Tox (3, 10, unit)	1m 11s
CI tests via Tox (3, 10, mypy)	34s
CI tests via Tox (3, 10, bandit)	13s
CI tests via Tox (3, 11, lint)	17s
CI tests via Tox (3, 11, unit)	1m 3s
CI tests via Tox (3, 11, mypy)	31s
CI tests via Tox (3, 11, bandit)	12s
CI tests via Tox (3, 12, lint)	15s
CI tests via Tox (3, 12, unit)	1m 39s
CI tests via Tox (3, 12, mypy)	41s
CI tests via Tox (3, 12, bandit)	21s
CI tests via Tox (3, 13, lint)	24s
CI tests via Tox (3, 13, unit)	1m 40s
CI tests via Tox (3, 13, mypy)	49s
CI tests via Tox (3, 13, bandit)	26s
	19m 3s

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