ci: add downgrade test #91

Job	Run time
doctests	57s
test (ubuntu-latest, x64, lts)	1m 18s
test (ubuntu-latest, x64, nightly)	1m 19s
test (ubuntu-latest, x86, lts)	1m 18s
test (ubuntu-latest, x86, nightly)	1m 19s
test (macos-13, x64, lts)	1m 20s
test (macos-13, x64, nightly)	1m 22s
test (windows-latest, x64, lts)	1m 27s
test (windows-latest, x64, nightly)	1m 12s
test (windows-latest, x86, lts)	1m 11s
test (windows-latest, x86, nightly)	1m 18s
test (macos-latest, aarch64, lts)	1m 19s
test (macos-latest, aarch64, nightly)	1m 14s
	16m 34s

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