HAMT iterators are not tracked by GC and can leak

[nascheme]

Bug report

Bug description:

The HAMT iterator types declare Py_TPFLAGS_HAVE_GC and implement tp_traverse, tp_clear and a tp_dealloc that untracks, but hamt_baseiter_new() never called PyObject_GC_Track(). A reference cycle running through an iterator -- iterator -> HAMT -> object -> iterator -- was therefore invisible to the collector and never freed. Because a contextvars.Context is backed by a HAMT that pins every object in it, one leaked iterator leaks the whole context.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155515: Use GC track for HAMT iterators #155517
• [3.15] gh-155515: Use GC tracking for HAMT iterators (GH-155517) #155697
• [3.14] gh-155515: Use GC tracking for HAMT iterators (GH-155517) #155698
• [3.13] gh-155515: Use GC tracking for HAMT iterators (GH-155517) #155918