Add `SchemaSerializer.reduce` method to enable `pickle` serialization #3747

Job	Run time
coverage	3m 47s
rust benchmarks	5m 43s
lint	2m 13s
build-wasm-emscripten	3m 56s
test-debug 3.11	1m 54s
test on ubuntu	3m 17s
test-debug pypy3.10	2m 31s
test-pydantic-integration	2m 27s
test on macos	3m 13s
test 3.7	3m 18s
test on windows	6m 43s
test 3.8	4m 13s
test 3.9	4m 36s
test 3.10	5m 0s
build sdist	43s
test 3.11	3m 38s
test 3.12-dev	4m 6s
test pypy3.7	4m 14s
build on ${{ matrix.platform \|\| matrix.os }} (${{ matrix.target }} - ${{ matrix.manylinux \|\| 'auto' }})	0s
test pypy3.8	4m 6s
build pgo-optimized on ${{ matrix.platform \|\| matrix.os }} (${{ matrix.interpreter }} - ${{ matrix.target }} - ${{ matrix.manylinux \|\| 'auto' }})	0s
test pypy3.9	4m 33s
test pypy3.10	4m 21s
test build on ${{ matrix.os }}	0s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
inspect-pypi-assets	0s
check	4s
release	0s
	1h 18m 36s

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