Some serialization of schemes #16626

Job	Run time
doctest (1.10, ubuntu-latest)	31m 6s
test (1.6, short, ubuntu-latest)	1h 29m 8s
test (1.6, long, ubuntu-latest)	1h 2m 4s
doctest (1.11-nightly, ubuntu-latest)	29m 20s
doctest (nightly, ubuntu-latest)	29m 10s
test (1.10, short, ubuntu-latest)	1h 13m 35s
test (1.10, long, ubuntu-latest)	54m 29s
doctest (1.10, ubuntu-latest, depwarn=error)	30m 34s
test (1.11-nightly, short, ubuntu-latest)	1h 19m 15s
doctest (1.10, macOS, RPTU)	12m 17s
test (1.11-nightly, long, ubuntu-latest)	1h 1m 16s
test (nightly, short, ubuntu-latest)	1h 23m 10s
test (nightly, long, ubuntu-latest)	59m 18s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 17m 46s
test (1.10, long, ubuntu-latest, depwarn=error)	55m 58s
test (1.10, short, macOS, RPTU)	13m 53s
test (1.10, long, macOS, RPTU)	14m 58s
test (1.10, book, ubuntu-latest)	55m 17s
	15h 12m 34s

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