add `AsRef` impls for `PyBackedStr` and `PyBackedBytes` #9130

Job	Run time
fmt	20s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	0s
check-feature-powerset	0s
careful	0s
check-msrv	11m 14s
coverage windows-latest	7s
docsrs	0s
coverage macos-14	6s
coverage ubuntu-latest	16m 11s
emscripten	0s
semver-checks	2m 42s
clippy/aarch64-apple-darwin/stable	10m 16s
test-cross-compilation ${{ matrix.os }} -> ${{ matrix.target }}	0s
test-cross-compilation-windows	0s
test-debug	11m 34s
valgrind	0s
test-version-limits	0s
clippy/x86_64-unknown-linux-gnu/stable	14m 6s
clippy/powerpc64le-unknown-linux-gnu/stable	13m 34s
clippy/s390x-unknown-linux-gnu/stable	13m 35s
clippy/wasm32-wasi/stable	13m 4s
clippy/x86_64-pc-windows-msvc/stable	17m 59s
clippy/i686-pc-windows-msvc/stable	17m 59s
clippy/x86_64-unknown-linux-gnu/beta	12m 25s
python3.12-x64 windows-latest rust-stable / build	11m 46s
python3.12-x64 macos-13 rust-stable / build	13m 9s
python3.12-x64 ubuntu-latest rust-stable / build	7m 46s
python3.12-arm64 macos-14 rust-stable / build	5m 13s
python3.12-x86 windows-latest rust-stable / build	10m 47s
python3.12-x64 ubuntu-latest rust-nightly / build	6m 26s
conclusion	1s
	3h 30m 20s

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