stats: fix Schema using deserialize to validate #1309

Job	Run time
Tests (acquisition, 3.9, pypi, postgresql14, opensearch2)	2m 46s
Tests (ill, 3.9, pypi, postgresql14, opensearch2)	4m 29s
Tests (document_requests, 3.9, pypi, postgresql14, opensearch2)	3m 17s
Tests (ils, 3.9, pypi, postgresql14, opensearch2)	18m 52s
Tests (circulation, 3.9, pypi, postgresql14, opensearch2)	8m 33s
	37m 57s

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