GH-129817: thread safety for tp_flags #2924

Job	Run time
Interpreter (Debug)	9m 3s
Free-Threaded (Debug) (19)	13m 29s
i686-pc-windows-msvc/msvc (Debug)	19m 48s
i686-pc-windows-msvc/msvc (Release)	10m 40s
x86_64-pc-windows-msvc/msvc (Debug)	18m 48s
x86_64-pc-windows-msvc/msvc (Release)	10m 38s
aarch64-pc-windows-msvc/msvc (Debug)	5m 8s
aarch64-pc-windows-msvc/msvc (Release)	6m 40s
x86_64-apple-darwin/clang (Debug)	11m 35s
x86_64-apple-darwin/clang (Release)	8m 7s
aarch64-apple-darwin/clang (Debug)	11m 15s
aarch64-apple-darwin/clang (Release)	11m 8s
x86_64-unknown-linux-gnu/gcc (Debug)	10m 29s
x86_64-unknown-linux-gnu/gcc (Release)	6m 0s
aarch64-unknown-linux-gnu/gcc (Debug)	9m 57s
aarch64-unknown-linux-gnu/gcc (Release)	7m 5s
	2h 49m 50s

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