Prefer symbols over strings as indeterminants #16753

Job	Run time
doctest (1.10, ubuntu-latest)	25m 35s
test (1.6, short, ubuntu-latest)	1h 27m 52s
doctest (1.11-nightly, ubuntu-latest)	25m 14s
test (1.6, long, ubuntu-latest)	1h 0m 18s
doctest (nightly, ubuntu-latest)	24m 15s
test (1.10, short, ubuntu-latest)	1h 22m 14s
doctest (1.10, ubuntu-latest, depwarn=error)	25m 13s
test (1.10, long, ubuntu-latest)	46m 43s
doctest (1.10, macOS, RPTU)	12m 22s
test (1.11-nightly, short, ubuntu-latest)	1h 26m 3s
test (1.11-nightly, long, ubuntu-latest)	54m 32s
test (nightly, short, ubuntu-latest)	1h 20m 0s
test (nightly, long, ubuntu-latest)	51m 20s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 9m 24s
test (1.10, long, ubuntu-latest, depwarn=error)	45m 44s
test (1.10, short, macOS, RPTU)	13m 36s
test (1.10, long, macOS, RPTU)	14m 40s
test (1.10, book, ubuntu-latest)	50m 57s
	14h 16m 2s

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