gh-117376: Fix off-by-ones in conversion functions (GH-124301) #1775

Job	Run time
Interpreter (Debug)	6m 47s
Free-Threaded (Debug)	10m 7s
i686-pc-windows-msvc/msvc (Debug)	19m 49s
i686-pc-windows-msvc/msvc (Release)	24m 11s
x86_64-pc-windows-msvc/msvc (Debug)	18m 9s
x86_64-pc-windows-msvc/msvc (Release)	20m 39s
aarch64-pc-windows-msvc/msvc (Debug)	5m 1s
aarch64-pc-windows-msvc/msvc (Release)	6m 36s
x86_64-apple-darwin/clang (Debug)	15m 28s
x86_64-apple-darwin/clang (Release)	20m 3s
aarch64-apple-darwin/clang (Debug)	8m 50s
aarch64-apple-darwin/clang (Release)	14m 22s
x86_64-unknown-linux-gnu/gcc (Debug)	7m 42s
x86_64-unknown-linux-gnu/gcc (Release)	17m 25s
x86_64-unknown-linux-gnu/clang (Debug)	7m 29s
x86_64-unknown-linux-gnu/clang (Release)	14m 23s
aarch64-unknown-linux-gnu/gcc (Debug)	1h 12m 40s
aarch64-unknown-linux-gnu/gcc (Release)	36m 36s
aarch64-unknown-linux-gnu/clang (Debug)	1h 6m 53s
aarch64-unknown-linux-gnu/clang (Release)	37m 54s
	7h 11m 4s

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