bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464) (GH-26465)

* _testcapi.heapgctype: implement a traverse function since the type
  is defined with Py_TPFLAGS_HAVE_GC.
* _decimal: PyDecSignalDictMixin_Type is no longer defined with
  Py_TPFLAGS_HAVE_GC since it has no traverse function.
(cherry picked from commit 142e5c5445c019542246d93fe2f9e195d3131686)

Co-authored-by: Victor Stinner <vstinner@python.org>

Co-authored-by: Victor Stinner <vstinner@python.org>