implement `Send` and `Sync` for `PyBackedStr` and `PyBackedBytes` #9166

Job	Run time
fmt	7s
clippy/${{ matrix.platform.rust-target }}/${{ matrix.rust }}	1s
semver-checks	1s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	1s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	1s
careful	1s
check-msrv	1s
check-feature-powerset	1s
coverage ${{ matrix.os }}	1s
valgrind	1s
docsrs	1s
emscripten	1s
test-cross-compilation ${{ matrix.os }} -> ${{ matrix.target }}	1s
test-debug	1s
test-version-limits	1s
test-cross-compilation-windows	1s
conclusion	1s
	23s

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