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/Doc/library/inspect.rst b/Doc/library/inspect.rst
index 850d601..1033964 100644
--- a/Doc/library/inspect.rst
+++ b/Doc/library/inspect.rst
@@ -95,6 +95,8 @@
 |           | __globals__       | global namespace in which |
 |           |                   | this function was defined |
 +-----------+-------------------+---------------------------+
+|           | __builtins__      | builtins namespace        |
++-----------+-------------------+---------------------------+
 |           | __annotations__   | mapping of parameters     |
 |           |                   | names to annotations;     |
 |           |                   | ``"return"`` key is       |
@@ -251,6 +253,10 @@
 
    Add ``cr_origin`` attribute to coroutines.
 
+.. versionchanged:: 3.10
+
+   Add ``__builtins__`` attribute to functions.
+
 .. function:: getmembers(object[, predicate])
 
    Return all the members of an object in a list of ``(name, value)``