ci: add downgrade test #90

Job	Run time
doctests	58s
test (ubuntu-latest, x64, lts)	1m 39s
test (ubuntu-latest, x64, nightly)	1m 43s
test (ubuntu-latest, x86, lts)	2m 35s
test (ubuntu-latest, x86, nightly)	2m 33s
test (macos-13, x64, lts)	2m 5s
test (macos-13, x64, nightly)	2m 35s
test (windows-latest, x64, lts)	2m 3s
test (windows-latest, x64, nightly)	2m 36s
test (windows-latest, x86, lts)	2m 40s
test (windows-latest, x86, nightly)	2m 39s
test (macos-latest, aarch64, lts)	1m 31s
test (macos-latest, aarch64, nightly)	1m 50s
	27m 27s

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