Some multiprocessing APIs should be possible to import or use even when multiprocessing itself isn't available

Bug report

Bug description:

Android doesn't support multiprocessing because it doesn't support the System V IPC API, and iOS doesn't support subprocesses at all. However, there are a couple of things that should still work on these platforms, but currently don't:

• multiprocessing.dummy.Pool (aka multiprocessing.pool.ThreadPool) currently depends on the multiprocessing synchronization primitives, when it could use the threading equivalents instead.

• multiprocessing.synchronize throws an exception on import if the synchronization primitives are unavailable. But this breaks some packages like joblib, which imports the primitives but doesn't always use them (joblib/joblib#825).

Chaquopy has some monkey patches to work around these issues, but it would be better if they were fixed upstream.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

Linked PRs

• gh-148050

まず multiprocessing.dummy.Pool と multiprocessing.pool.ThreadPool のエントリーポイントを確認し、次に multiprocessing.synchronize と、同期プリミティブを持たないプラットフォームでのインポート時の失敗を調査します。必要な動作を threading の同等機能と比較し、リンクされた PR gh-148050 を確認します。対応する API を、利用できない multiprocessing 機能なしでインポートまたは使用できれば完了です。