Fix a soundness bug with `PyClassInitializer` #10863

Job	Run time
fmt	16s
resolve	4s
check-msrv	11m 9s
semver-checks	37s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	0s
clippy/aarch64-apple-darwin/stable	11m 1s
valgrind	0s
careful	0s
coverage windows-latest	3s
coverage macos-14	6s
clippy/x86_64-unknown-linux-gnu/stable	13m 42s
coverage ubuntu-latest	11m 8s
docsrs	0s
clippy/powerpc64le-unknown-linux-gnu/stable	13m 8s
emscripten	0s
clippy/s390x-unknown-linux-gnu/stable	13m 13s
test-debug	0s
clippy/wasm32-wasi/stable	13m 13s
clippy/x86_64-pc-windows-msvc/stable	19m 10s
test-free-threaded	0s
clippy/i686-pc-windows-msvc/stable	16m 8s
test-version-limits	0s
clippy/x86_64-unknown-linux-gnu/beta	13m 32s
check-feature-powerset ${{ matrix.rust }}	0s
test-cross-compilation ${{ matrix.os }} -> ${{ matrix.target }}	0s
test-cross-compilation-windows	0s
python3.12-x64 ubuntu-latest rust-nightly / build	6m 23s
python3.12-x64 macos-13 rust-stable / build	11m 55s
python3.12-arm64 macos-14 rust-stable / build	6m 37s
python3.12-x64 ubuntu-latest rust-stable / build	9m 40s
python3.12-x86 windows-latest rust-stable / build	10m 41s
python3.12-x64 windows-latest rust-stable / build	14m 36s
conclusion	1s
	3h 16m 23s

Provide feedback