bpo-42990: Add __builtins__ attribute to functions (GH-24559)

Expose the new PyFunctionObject.func_builtins member in Python as a
new __builtins__ attribute on functions.

Document also the behavior change in What's New in Python 3.10.
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index befb7ab..54a4cbe 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -682,9 +682,10 @@ class NewPoint(tuple):
         self.assertEqual(np.y, 2)
 
     def test_new_builtins_issue_43102(self):
-        self.assertEqual(
-            namedtuple('C', ()).__new__.__globals__['__builtins__'],
-            {})
+        obj = namedtuple('C', ())
+        new_func = obj.__new__
+        self.assertEqual(new_func.__globals__['__builtins__'], {})
+        self.assertEqual(new_func.__builtins__, {})
 
 
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