introduce `SubPcGroup` #14781

Job	Run time
doctest (1.10, ubuntu-latest)	27m 46s
doctest (1.11-nightly, ubuntu-latest)	27m 15s
test (1.6, short, ubuntu-latest)	1h 2m 21s
doctest (nightly, ubuntu-latest)	26m 50s
test (1.6, long, ubuntu-latest)	37m 58s
doctest (1.10, ubuntu-latest, depwarn=error)	26m 58s
test (1.10, short, ubuntu-latest)	50m 59s
doctest (1.10, macOS, RPTU)	17m 24s
test (1.10, long, ubuntu-latest)	32m 19s
test (1.11-nightly, short, ubuntu-latest)	55m 37s
test (1.11-nightly, long, ubuntu-latest)	31m 51s
test (nightly, short, ubuntu-latest)	56m 31s
test (nightly, long, ubuntu-latest)	34m 25s
test (1.10, short, ubuntu-latest, depwarn=error)	47m 47s
test (1.10, long, ubuntu-latest, depwarn=error)	31m 14s
test (1.10, short, macOS, RPTU)	17m 45s
test (1.10, long, macOS, RPTU)	16m 26s
test (1.10, book, ubuntu-latest)	31m 18s
	10h 32m 44s

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