gh-119180: PEP 649: Add __annotate__ attributes

[JelleZijlstra]

This implements https://peps.python.org/pep-0649/#annotate-and-annotations. In separate PRs I will work on making the compiler synthesize __annotate__ functions.

• Issue: Implement PEP 649 and PEP 749 #119180

[picnixz]

Some comments. I'm wondering whether the error message "annotate returned a non-dict" could be changed to "annotate must return a dict" or "did not return a dict"

[JelleZijlstra]

Some comments. I'm wondering whether the error message "annotate returned a non-dict" could be changed to "annotate must return a dict" or "did not return a dict"

For comparison:

>>> iter(X()) Traceback (most recent call last): File "<python-input-4>", line 1, in <module> iter(X()) ~~~~^^^^^ TypeError: iter() returned non-iterator of type 'int' 

I think I'll make the __annotate__ errors similar.

[carljm]

Looks good in general, a few comments.

I would prefer to remove the "set __annotate__ to None" behavior; I guess we would have to ask the Steering Council about that. Maybe not worth it unless we see it causing confusion in practice.