Prepare 21.0.0 release. #1431

Job	Run time
mypy	24s
sdist	1m 2s
ruff	8s
parse_sdl (macos-latest, 3.2.16)	25s
parse_sdl (windows-latest, 3.2.16)	1m 2s
tox (ubuntu-latest)	1m 59s
test-docs	2m 8s
build-macos (cp310-*_universal2)	2m 28s
pyodide	2m 47s
build-macos (pp310-*)	4m 49s
build-macos (cp314t-*_universal2)	2m 14s
linux-wheels (aarch64, pp310-manylinux*)	3m 51s
linux-wheels (x86_64, cp310-manylinux*)	4m 8s
linux-wheels (x86_64, cp314t-manylinux*)	4m 21s
build (ubuntu-latest, 3.10, x64)	3m 30s
linux-wheels (aarch64, cp314t-manylinux*)	4m 14s
linux-wheels (x86_64, pp310-manylinux*)	3m 34s
build (windows-latest, 3.10, x64)	2m 53s
build (windows-latest, 3.10, x86)	4m 47s
build (windows-latest, pypy-3.10, x64)	3m 55s
build (ubuntu-latest, 3.14t, x64)	3m 40s
build (windows-11-arm, 3.14t, arm64)	5m 11s
build (ubuntu-latest, pypy-3.10, x64)	3m 37s
build (windows-11-arm, 3.11, arm64)	4m 36s
build (windows-latest, 3.14t, x64)	6m 20s
build (windows-latest, 3.14t, x86)	4m 29s
linux-wheels (aarch64, cp310-manylinux*)	4m 5s
publish	0s
	1h 26m 37s

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