…e attributes

Fixes python#21716.

When mypyc compiles a subclass of a builtin type (`Exception` or `dict`)
with instance attributes on Python <3.12, it appends a __dict__ slot at
tp_dictoffset = sizeof(builtin_base) to hold them. But it emits no
subtype-specific tp_dealloc, so the type inherits the base's dealloc which
knows nothing about the extra slot. That dict (and everything it
references) leaks on every instance destruction.

Hit downstream in sqlglot[c] (tobymao/sqlglot#7853) where every
raised-and-caught ParseError leaked ~1 KB. On 3.12+ mypyc already avoids
adding the extra offset, so only <3.12 is affected.

Changes in `mypyc/codegen/emitclass.py`:

* Extend the gate that emits tp_dealloc/tp_clear/tp_traverse (and
  Py_TPFLAGS_HAVE_GC) to fire for any builtin_base subclass with
  has_dict under capi < 3.12. The existing builtin-base dealloc dance
  handles the flow.

* Set Py_TPFLAGS_HAVE_GC explicitly. PyType_Ready only inherits HAVE_GC
  from the base when it also inherits both tp_traverse and tp_dealloc;
  once we supply our own, that inheritance is lost.

* Fix a latent offset bug in generate_traverse_for_class /
  generate_clear_for_class: they used sizeof(struct_name) for the extra
  dict slot, but for builtin_base classes tp_dictoffset is
  sizeof(builtin_base). Previously unreachable because the emission
  gate never fired for builtin_base classes.

Changes in `mypyc/codegen/emit.py` (`emit_base_tp_function_call`):

* Walk the tp_base chain past any Py_TPFLAGS_HEAPTYPE ancestors before
  delegating tp_dealloc/tp_traverse/tp_clear. Required for Exception
  subclasses that inherit through an interpreted Python heap type (e.g.
  `argparse.ArgumentTypeError` in mypy's own VersionTypeError): calling
  the heap type's tp_dealloc dispatches through CPython's
  subtype_dealloc, which reads Py_TYPE(self) (still our subtype), finds
  our own tp_dealloc, and calls it back — infinite recursion crashing
  the compiled binary on 3.10/3.11. Our subtype_clear already walks the
  full MRO and clears all ancestors' fields, so calling the static
  ancestor's tp_dealloc directly is safe and correct.

Regression tests:

* testSubclassExceptionNoAttributeLeak (new): a Payload class with a
  __del__ counter detects the leak deterministically — 1000 fresh
  exceptions each hold a fresh payload; without the fix all 1000
  payloads survive.
* testDelForDictSubclass (updated): the stale
  `sys.version_info >= (3, 12)` guard is dropped since dict subclass
  __del__ now works correctly on all Python versions.

When a mypyc-compiled Exception subclass inherits from an interpreted
base that defines __del__, our custom tp_dealloc bypasses
subtype_dealloc, so the inherited tp_finalize was never invoked.
Runtime-gate on Py_TYPE(self)->tp_finalize and call
PyObject_CallFinalizerFromDealloc for builtin_base classes.