tests

Add serialization stubs #507

Job	Run time
Tests (8.1, ubuntu-latest, prefer-lowest)	25s
Tests (8.1, ubuntu-latest, prefer-stable)	16s
Tests (8.1, macos-latest, prefer-lowest)	1m 22s
Tests (8.1, macos-latest, prefer-stable)	1m 22s
Tests (8.1, windows-latest, prefer-lowest)	41s
Tests (8.1, windows-latest, prefer-stable)	37s
Tests (8.2, ubuntu-latest, prefer-lowest)	20s
Tests (8.2, ubuntu-latest, prefer-stable)	15s
Tests (8.2, macos-latest, prefer-lowest)	1m 32s
Tests (8.2, macos-latest, prefer-stable)	1m 25s
Tests (8.2, windows-latest, prefer-lowest)	34s
Tests (8.2, windows-latest, prefer-stable)	43s
Tests (8.3, ubuntu-latest, prefer-lowest)	13s
Tests (8.3, ubuntu-latest, prefer-stable)	14s
Tests (8.3, macos-latest, prefer-lowest)	1m 28s
Tests (8.3, macos-latest, prefer-stable)	1m 24s
Tests (8.3, windows-latest, prefer-lowest)	33s
Tests (8.3, windows-latest, prefer-stable)	49s
	14m 13s

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