Implement smarter `is_subtype` logic #1326

Job	Run time
CI linters via Tox (lintreadme)	53s
CI tests via Tox (3, 8, lint)	26s
CI linters via Tox (pydocstyle)	24s
build_test_container	1m 50s
Schema Salad release test	1m 50s
CI tests via Tox (3, 8, bandit)	28s
CI tests via Tox (3, 8, unit)	1m 2s
CI tests via Tox (3, 8, mypy)	1m 11s
CI tests via Tox (3, 8, memleak)	1m 10s
CI tests via Tox (3, 9, lint)	30s
CI tests via Tox (3, 9, bandit)	26s
CI tests via Tox (3, 9, unit)	28s
CI tests via Tox (3, 9, mypy)	45s
CI tests via Tox (3, 9, memleak)	41s
CI tests via Tox (3, 10, lint)	24s
CI tests via Tox (3, 10, bandit)	38s
CI tests via Tox (3, 10, unit)	1m 52s
CI tests via Tox (3, 10, mypy)	1m 52s
CI tests via Tox (3, 10, memleak)	1m 25s
CI tests via Tox (3, 11, lint)	31s
CI tests via Tox (3, 11, bandit)	25s
CI tests via Tox (3, 11, unit)	1m 49s
CI tests via Tox (3, 11, mypy)	1m 51s
CI tests via Tox (3, 11, memleak)	1m 22s
CI tests via Tox (3, 12, lint)	33s
CI tests via Tox (3, 12, bandit)	38s
CI tests via Tox (3, 12, unit)	1m 22s
CI tests via Tox (3, 12, mypy)	1m 10s
	27m 56s

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