make _PyGenObject_HEAD private, etc #61

Job	Run time
Interpreter (Debug)	6m 28s
Free-Threaded (Debug)	9m 22s
i686-pc-windows-msvc/msvc (Debug)	16m 26s
i686-pc-windows-msvc/msvc (Release)	19m 39s
x86_64-pc-windows-msvc/msvc (Debug)	16m 15s
x86_64-pc-windows-msvc/msvc (Release)	20m 23s
aarch64-pc-windows-msvc/msvc (Debug)	4m 53s
aarch64-pc-windows-msvc/msvc (Release)	6m 46s
x86_64-apple-darwin/clang (Debug)	10m 46s
x86_64-apple-darwin/clang (Release)	13m 6s
aarch64-apple-darwin/clang (Debug)	9m 7s
aarch64-apple-darwin/clang (Release)	13m 54s
x86_64-unknown-linux-gnu/gcc (Debug)	7m 28s
x86_64-unknown-linux-gnu/gcc (Release)	8m 44s
x86_64-unknown-linux-gnu/clang (Debug)	7m 42s
x86_64-unknown-linux-gnu/clang (Release)	8m 13s
aarch64-unknown-linux-gnu/gcc (Debug)	1h 8m 43s
aarch64-unknown-linux-gnu/gcc (Release)	46m 55s
aarch64-unknown-linux-gnu/clang (Debug)	1h 1m 3s
aarch64-unknown-linux-gnu/clang (Release)	35m 21s
	6h 31m 14s

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