gh-132426: Add get_annotate_from_class_namespace replacing get_annotate_function...

gh-132426: Add get_annotate_from_class_namespace replacing get_annotate_function (#132490)

As noted on the issue, making get_annotate_function() support both types and
mappings is problematic because one object may be both. So let's add a new one
that works with any mapping.

This leaves get_annotate_function() not very useful, so remove it.