Fix ABC registration for SetProxy in multiprocessing.managers

Register _BaseSetProxy as MutableSet instead of MutableMapping.
SetProxy implements set semantics and does not implement the mapping
protocol, so the previous ABC registration caused incorrect type
classification.

Signed-off-by: Yongtao Huang <yongtaoh2022@gmail.com>

Author: hyongtao-code

Lib/multiprocessing/managers.py

@@ -1224,7 +1224,7 @@ def __isub__(self, value):
 
     __class_getitem__ = classmethod(types.GenericAlias)
 
-collections.abc.MutableMapping.register(_BaseSetProxy)
+collections.abc.MutableSet.register(_BaseSetProxy)
 
 
 ArrayProxy = MakeProxyType('ArrayProxy', (