Convert Tileset into a MutableMapping #1403

Job	Run time
sdist	1m 5s
ruff	7s
mypy	27s
parse_sdl (macos-latest, 3.2.16)	26s
parse_sdl (windows-latest, 3.2.16)	37s
test-docs	2m 5s
pyodide	2m 35s
build-macos (cp314t-*_universal2)	2m 21s
tox (ubuntu-latest)	2m 8s
build-macos (pp310-*)	3m 37s
linux-wheels (x86_64, pp310-manylinux*)	3m 32s
linux-wheels (x86_64, cp310-manylinux*)	5m 31s
build-macos (cp310-*_universal2)	2m 32s
linux-wheels (aarch64, cp310-manylinux*)	4m 37s
linux-wheels (aarch64, pp310-manylinux*)	3m 45s
linux-wheels (x86_64, cp314t-manylinux*)	3m 58s
linux-wheels (aarch64, cp314t-manylinux*)	4m 8s
build (ubuntu-latest, 3.10, x64)	3m 34s
build (windows-latest, 3.14t, x86)	4m 0s
build (windows-latest, 3.10, x64)	3m 0s
build (ubuntu-latest, pypy-3.10, x64)	3m 0s
build (windows-latest, pypy-3.10, x64)	3m 37s
build (windows-11-arm, 3.14t, arm64)	5m 8s
build (windows-11-arm, 3.11, arm64)	14m 37s
build (ubuntu-latest, 3.14t, x64)	3m 49s
build (windows-latest, 3.14t, x64)	3m 37s
build (windows-latest, 3.10, x86)	3m 28s
publish	1s
	1h 31m 22s

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