gh-148608: Unbind classmethods in LOAD_ATTR_CLASS #2867

Job	Run time
aarch64-unknown-linux-gnu/gcc	6m 58s
aarch64-apple-darwin/clang	6m 39s
x86_64-unknown-linux-gnu/gcc-free-threading	6m 29s
x86_64-apple-darwin/clang	6m 27s
x86_64-unknown-linux-gnu/gcc	6m 59s
	33m 32s

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