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multiprocessing.reduction still appears to use DuplicateHandle to copy sockets.
I propose adding a pair of custom functions to _multiprocessing, that "pickles" and "unpickles" handles. It can detect socket handles as being different from e.g. pipe handles by using WSADuplicateSocket and return a bytes object, similar to what is already done in socketmodule (see issue 14310)
On non-windows, this would be a no-op.
_multiprocessing already linkes with winsock, whereas the subprocess is part of python core which doesn't.