gh-129948: Add `set()` to `multiprocessing.managers.SyncManager` (#129949)

The SyncManager provided support for various data structures such as dict, list, and queue, but oddly, not set.
This introduces support for set by defining SetProxy and registering it with SyncManager.

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>