Python APIの`Synthesizer`を`close()`可能にする #1909

Job	Run time
actionlint	18s
build-and-test-python-api (windows-latest)	8m 59s
build-and-test-python-api (macos-latest)	6m 54s
build-and-test-python-api (ubuntu-latest)	4m 4s
build-windows-cpp-example	5m 18s
build-unix-cpp-example (macos-latest, osx-x64-cpu-cpp-shared)	6m 39s
build-unix-cpp-example (ubuntu-latest, linux-x64-cpu-cpp-shared)	3m 9s
rust-integration-test-strategy-matrix	2s
c-header	1m 30s
rust-lint	2m 1s
rust-unit-test	3m 15s
validate-cargo-lock	34s
shellcheck	17s
rust-integration-test (windows-2019, directml)	4m 2s
rust-integration-test (macos-11)	2m 39s
rust-integration-test (ubuntu-20.04)	1m 30s
	51m 11s

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