gh-144981: Make PyUnstable_Code_SetExtra/GetExtra thread-safe #2252

Job	Run time
x86_64-pc-windows-msvc/msvc-free-threading	7m 26s
x86_64-apple-darwin/clang	14m 32s
aarch64-apple-darwin/clang	8m 31s
x86_64-unknown-linux-gnu/gcc-free-threading	8m 32s
x86_64-unknown-linux-gnu/gcc	9m 47s
x86_64-pc-windows-msvc/msvc	9m 56s
aarch64-unknown-linux-gnu/gcc	7m 9s
	1h 5m 53s

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