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

Comienza revisando los puntos de entrada multiprocessing.dummy.Pool y multiprocessing.pool.ThreadPool; después, inspecciona multiprocessing.synchronize y su fallo durante la importación en plataformas sin primitivas de sincronización. Compara el comportamiento necesario con los equivalentes de threading y revisa el PR enlazado gh-148050; se considera terminado cuando las APIs compatibles se puedan importar o usar sin instalaciones de multiprocessing no disponibles.