We're out out of type flags, let's add new API for new ones

We almost ran out of type flag bits. Let’s add more space in 3.16, and while we’re at it, decouple the public API from the internal storage (kinda what PyType_Spec/PySlot do for PyTypeObject struct itself). That way, we can later rearrange the flags or remove obsolete ones… except for the public ones we have in tp_flags now. I’d like to have a design ready before we need to add the next flag, so we don’t need to rush at that point. Proposal: For storage, let’s start with uint8_t _tp_extra_fla...