[#528] Introduce new warning for packed unions which has no packable … #323

Job	Run time
build_windows (windows-latest, 3.8)	4h 14m 57s
build_windows (windows-latest, 3.9)	5h 6m 21s
build_windows (windows-latest, 3.10)	5h 57m 9s
build_windows (windows-latest, 3.11)	4h 52m 25s
	20h 10m 52s

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