Convert Tileset into a MutableMapping #1404

Job	Run time
mypy	22s
ruff	9s
sdist	1m 0s
parse_sdl (windows-latest, 3.2.16)	43s
parse_sdl (macos-latest, 3.2.16)	32s
pyodide	2m 50s
tox (ubuntu-latest)	2m 23s
test-docs	2m 20s
build-macos (pp310-*)	4m 1s
build-macos (cp310-*_universal2)	2m 24s
linux-wheels (aarch64, cp310-manylinux*)	4m 39s
build-macos (cp314t-*_universal2)	3m 52s
linux-wheels (x86_64, cp314t-manylinux*)	4m 7s
linux-wheels (x86_64, cp310-manylinux*)	4m 27s
linux-wheels (aarch64, cp314t-manylinux*)	4m 15s
linux-wheels (aarch64, pp310-manylinux*)	3m 38s
linux-wheels (x86_64, pp310-manylinux*)	3m 46s
build (windows-latest, pypy-3.10, x64)	3m 43s
build (ubuntu-latest, 3.10, x64)	4m 16s
build (ubuntu-latest, 3.14t, x64)	3m 45s
build (windows-latest, 3.14t, x64)	3m 49s
build (windows-latest, 3.10, x86)	3m 36s
build (windows-11-arm, 3.14t, arm64)	4m 53s
build (windows-latest, 3.10, x64)	2m 47s
build (ubuntu-latest, pypy-3.10, x64)	3m 4s
build (windows-latest, 3.14t, x86)	3m 40s
build (windows-11-arm, 3.11, arm64)	3m 53s
publish	0s
	1h 22m 54s

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