Make sure to properly untrack gc objects before freeing them (#4461)

* Make sure to properly untrack gc objects before freeing them

* style: pre-commit fixes

* Fix lint

* Add comment about where the original track comes from

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: albanD

include/pybind11/detail/class.h

@@ -445,9 +445,17 @@ inline void clear_instance(PyObject *self) {
 /// Instance destructor function for all pybind11 types. It calls `type_info.dealloc`
 /// to destroy the C++ object itself, while the rest is Python bookkeeping.
 extern "C" inline void pybind11_object_dealloc(PyObject *self) {
+    auto *type = Py_TYPE(self);
+
+    // If this is a GC tracked object, untrack it first
+    // Note that the track call is implicitly done by the
+    // default tp_alloc, which we never override.
+    if (PyType_HasFeature(type, Py_TPFLAGS_HAVE_GC) != 0) {
+        PyObject_GC_UnTrack(self);
+    }
+
     clear_instance(self);
 
-    auto *type = Py_TYPE(self);
     type->tp_free(self);
 
 #if PY_VERSION_HEX < 0x03080000