Remove `Generator` in `_purge_unreferenced_state_groups` twice #3563

Job	Run time
Calculate list of debian distros	5s
Build wheels on ubuntu-20.04 for x86_64	1m 51s
Build sdist	0s
Build .deb packages (debian:sid)	9m 37s
Attach assets to release	0s
	11m 33s

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