gh-111178: fix UBSan failures in `Modules/_functoolsmodule.c` (GH-129778)

Fix UBSan failures for `keyobject`, `lru_list_elem`, `lru_cache_object`.
Suppress unused return values.
Change `_PyPartialObject_CAST` (from #124733) to `partialobject_CAST`
for consistency with the current style for these macros