[3.12] gh-121137: Add missing Py_DECREF calls for ADDITEMS opcode of _pickle.c (GH-121136) (#121140)

gh-121137: Add missing Py_DECREF calls for ADDITEMS opcode of _pickle.c (GH-121136)

PyObject_GetAttr returns a new reference, but this reference is never decremented using Py_DECREF, so Py_DECREF calls to this referece are added
(cherry picked from commit 92893fd8dc803ed7cdde55d29d25f84ccb5e3ef0)

Co-authored-by: Justin Applegate <70449145+Legoclones@users.noreply.github.com>