ci: add downgrade test #89

Job	Run time
doctests	57s
test (ubuntu-latest, x64, lts)	1m 37s
test (ubuntu-latest, x64, nightly)	1m 43s
test (ubuntu-latest, x86, lts)	2m 46s
test (ubuntu-latest, x86, nightly)	2m 44s
test (macos-13, x64, lts)	1m 57s
test (macos-13, x64, nightly)	2m 50s
test (windows-latest, x64, lts)	2m 1s
test (windows-latest, x64, nightly)	2m 51s
test (windows-latest, x86, lts)	3m 58s
test (windows-latest, x86, nightly)	3m 31s
test (macos-latest, aarch64, lts)	1m 34s
test (macos-latest, aarch64, nightly)	1m 44s
	30m 13s

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