bpo-40217: Ensure Py_VISIT(Py_TYPE(self)) is always called for PyType_FromSpec types (reverts GH-19414) (GH-20264)

Heap types now always visit the type in tp_traverse. See added docs for details.

This reverts commit 0169d3003be3d072751dd14a5c84748ab63a249f.

Automerge-Triggered-By: @encukou