RFC: remove private `_Py` symbols from `pyo3-ffi` #11090

Job	Run time
fmt	53s
resolve	4s
check-msrv	7m 7s
semver-checks	2m 42s
clippy/aarch64-apple-darwin/stable	6m 37s
careful	0s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	0s
clippy/x86_64-unknown-linux-gnu/stable	5m 31s
clippy/powerpc64le-unknown-linux-gnu/stable	6m 46s
clippy/s390x-unknown-linux-gnu/stable	6m 35s
coverage windows-latest	1m 57s
docsrs	0s
coverage macos-14	1m 15s
clippy/wasm32-wasi/stable	6m 27s
coverage ubuntu-latest	1m 15s
clippy/x86_64-pc-windows-msvc/stable	8m 21s
clippy/i686-pc-windows-msvc/stable	4m 59s
valgrind	0s
clippy/x86_64-unknown-linux-gnu/beta	7m 4s
check-feature-powerset ${{ matrix.rust }}	0s
emscripten	0s
test-cross-compilation ${{ matrix.os }} -> ${{ matrix.target }}	0s
test-cross-compilation-windows	0s
test-debug	0s
test-free-threaded	0s
test-version-limits	0s
python3.12-x64 ubuntu-latest rust-nightly / build	1s
python3.12-x64 macos-13 rust-stable / build	1s
python3.12-arm64 macos-14 rust-stable / build	1m 4s
python3.12-x64 ubuntu-latest rust-stable / build	1s
python3.12-x86 windows-latest rust-stable / build	1s
python3.12-x64 windows-latest rust-stable / build	1s
conclusion	1s
	1h 8m 43s

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