Memory leak on interpreter shutdown when reference cycle exists between `structseq` type and its instance

Bug report

Bug description:

Environment

• Commit/Branch: main (rev: 2d9fb5f2f70b18ce9e5d9f7f76fd5ef3f118d838)
• OS: Linux
• Build flags:

  CC=clang CXX=clang++ LDFLAGS='-fuse-ld=lld' ./configure --with-address-sanitizer --with-undefined-behavior-sanitizer --with-pydebug && make -j$(nproc)
  

Steps to reproduce

Run:

./python -c "import time; t = time.gmtime(); type(t).refcyle = t;"

Note: Explicitly breaking the cycle avoids the leak:

./python -c "import time; t = time.gmtime(); type(t).refcyle = t; del type(t).refcyle"

Expected behavior

The garbage collector or interpreter finalization breaks the reference cycle during shutdown; no memory leaks reported by LeakSanitizer.

Actual behavior

LeakSanitizer detects memory leaks (indirect leaks of the type, its dict, descriptors, and the instance):

=================================================================
==1164172==ERROR: LeakSanitizer: detected memory leaks

Indirect leak of 1472 byte(s) in 1 object(s) allocated from:
    #0 0x55571ba2f651 in malloc
    #1 0x55571c0c8db7 in _PyMem_DebugRawAlloc Objects/obmalloc.c:3103:24
    #2 0x55571c130728 in _PyObject_MallocWithType Include/internal/pycore_object_alloc.h:46:17
    #3 0x55571c130728 in _PyType_AllocNoTrack Objects/typeobject.c:2495:19
    #4 0x55571c13040d in PyType_GenericAlloc Objects/typeobject.c:2526:21
    #5 0x55571c1357f9 in type_from_slots_or_spec Objects/typeobject.c:5591:30
    #6 0x55571c1148ec in _PyStructSequence_NewType Objects/structseq.c:780:28
    #7 0x55571ca6ea5c in time_exec Modules/timemodule.c:2113:31
...
Indirect leak of 176 byte(s) in 1 object(s) allocated from:
    #0 0x55571ba2f651 in malloc
    #1 0x55571c0c8db7 in _PyMem_DebugRawAlloc Objects/obmalloc.c:3103:24
    #2 0x55571c5d4028 in _PyObject_MallocWithType Include/internal/pycore_object_alloc.h:46:17
    #3 0x55571c5d4028 in gc_alloc Python/gc.c:2013:17
    #4 0x55571c5d4440 in _PyObject_GC_NewVar Python/gc.c:2055:25
    #5 0x55571c110247 in PyStructSequence_New Objects/structseq.c:77:11
    #6 0x55571ca69e1d in tmtotuple Modules/timemodule.c:463:19
    #7 0x55571ca67616 in time_gmtime Modules/timemodule.c:541:12
...
SUMMARY: AddressSanitizer: 6946 byte(s) leaked in 50 allocation(s).

Full LeakSanitizer output - logs.txt

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-157179

Start with the reproducer under the ASan/UBSan pydebug build, then inspect Objects/structseq.c and the allocation paths in Objects/typeobject.c, Python/gc.c, and Modules/timemodule.c. Compare the behavior with linked PR gh-157179. Done means the cycle no longer produces LeakSanitizer leaks during interpreter shutdown.