Convert Tileset into a MutableMapping #1405

Job	Run time
mypy	23s
sdist	49s
ruff	9s
parse_sdl (macos-latest, 3.2.16)	32s
parse_sdl (windows-latest, 3.2.16)	39s
tox (ubuntu-latest)	2m 16s
build-macos (cp310-*_universal2)	2m 54s
build-macos (pp310-*)	4m 14s
test-docs	2m 5s
build-macos (cp314t-*_universal2)	2m 12s
linux-wheels (x86_64, cp310-manylinux*)	4m 8s
linux-wheels (aarch64, cp314t-manylinux*)	4m 16s
pyodide	3m 52s
linux-wheels (aarch64, cp310-manylinux*)	4m 4s
linux-wheels (aarch64, pp310-manylinux*)	3m 37s
build (windows-latest, 3.14t, x64)	3m 15s
linux-wheels (x86_64, pp310-manylinux*)	3m 35s
build (windows-latest, pypy-3.10, x64)	3m 34s
linux-wheels (x86_64, cp314t-manylinux*)	4m 8s
build (ubuntu-latest, pypy-3.10, x64)	3m 11s
build (ubuntu-latest, 3.14t, x64)	4m 14s
build (windows-latest, 3.14t, x86)	3m 32s
build (windows-11-arm, 3.11, arm64)	5m 27s
build (windows-latest, 3.10, x64)	2m 58s
build (windows-latest, 3.10, x86)	4m 32s
build (windows-11-arm, 3.14t, arm64)	4m 39s
build (ubuntu-latest, 3.10, x64)	3m 39s
publish	0s
	1h 22m 54s

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