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.

Lib/annotationlib.py

@@ -12,7 +12,7 @@ __all__ = [
     "ForwardRef",
     "call_annotate_function",
     "call_evaluate_function",
-    "get_annotate_function",
+    "get_annotate_from_class_namespace",
     "get_annotations",
     "annotations_to_string",
     "type_repr",
@@ -619,20 +619,16 @@ def call_annotate_function(annotate, format, *, owner=None, _is_evaluate=False):
         raise ValueError(f"Invalid format: {format!r}")
 
 
-def get_annotate_function(obj):
-    """Get the __annotate__ function for an object.
+def get_annotate_from_class_namespace(obj):
+    """Retrieve the annotate function from a class namespace dictionary.
 
-    obj may be a function, class, or module, or a user-defined type with
-    an `__annotate__` attribute.
-
-    Returns the __annotate__ function or None.
+    Return None if the namespace does not contain an annotate function.
+    This is useful in metaclass ``__new__`` methods to retrieve the annotate function.
     """
-    if isinstance(obj, dict):
-        try:
-            return obj["__annotate__"]
-        except KeyError:
-            return obj.get("__annotate_func__", None)
-    return getattr(obj, "__annotate__", None)
+    try:
+        return obj["__annotate__"]
+    except KeyError:
+        return obj.get("__annotate_func__", None)
 
 
 def get_annotations(
@@ -832,7 +828,7 @@ def _get_and_call_annotate(obj, format):
 
     May not return a fresh dictionary.
     """
-    annotate = get_annotate_function(obj)
+    annotate = getattr(obj, "__annotate__", None)
     if annotate is not None:
         ann = call_annotate_function(annotate, format, owner=obj)
         if not isinstance(ann, dict):