gh-111178: Avoid calling functions from incompatible pointer types in descrobject.c (GH-112861)

Fix undefined behavior warnings (UBSan  -fsanitize=function), for example:

Python/generated_cases.c.h:3315:13: runtime error: call to function mappingproxy_dealloc through pointer to incorrect function type 'void (*)(struct _object *)'
descrobject.c:1160: note: mappingproxy_dealloc defined here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior Python/generated_cases.c.h:3315:13 in