Fix minor type error. #735

Job	Run time
isort	7s
black	6s
mypy	23s
sdist	44s
test-docs	2m 13s
linux-wheels (x86_64, cp37-manylinux*)	3m 3s
linux-wheels (x86_64, pp37-manylinux*)	3m 13s
linux-wheels (aarch64, cp37-manylinux*)	14m 58s
linux-wheels (aarch64, pp37-manylinux*)	12m 47s
build-macos (cp38-*_universal2)	4m 47s
build-macos (cp38-*_x86_64)	4m 6s
build-macos (cp38-*_arm64)	3m 24s
build-macos (pp37-*)	3m 28s
build (ubuntu-latest, 3.7, x64)	4m 44s
build (ubuntu-latest, 3.8, x64)	4m 39s
build (ubuntu-latest, 3.9, x64)	3m 46s
build (ubuntu-latest, pypy-3.7, x64)	4m 27s
build (windows-latest, 3.7, x64)	3m 58s
build (windows-latest, 3.8, x64)	3m 48s
build (windows-latest, 3.9, x64)	4m 22s
build (windows-latest, pypy-3.7, x64)	14m 3s
build (windows-latest, 3.7, x86)	5m 37s
build (windows-latest, pypy-3.7, x86)	11m 51s
parse_sdl (windows-latest, 2.0.14)	47s
parse_sdl (windows-latest, 2.0.16)	46s
parse_sdl (macos-latest, 2.0.14)	44s
parse_sdl (macos-latest, 2.0.16)	32s
isolated (ubuntu-latest)	2m 8s
isolated (windows-latest)	1m 56s
publish	0s
	2h 1m 27s

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