gh-119180: Alternative approach to metaclass annotations: never use the dict

[JelleZijlstra]

Like #120719, this is an experimental way to fix the issues discussed in python/peps#3847.

This PR takes the approach of "never use the class dict". Instead, we repurpose the unused tp_cache field to hold annotation information. It can hold any of:

• NULL: the class has no annotations
• a dict: the class has __annotations__, but no __annotate__
• a callable: the class has only __annotate__
• a tuple: the class has both annotate and annotations

• Issue: Implement PEP 649 and PEP 749 #119180

[JelleZijlstra]

We'll go with #122074.