bpo-42308: Add threading.__excepthook__ (GH-23218)

Add threading.__excepthook__ to allow retrieving the original value
of threading.excepthook in case it is set to a broken or a different
value.
diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index e05486f..6907354 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -71,6 +71,13 @@
 
    .. versionadded:: 3.8
 
+.. data:: __excepthook__
+
+   Holds the original value of :func:`threading.excepthook`. It is saved so that the
+   original value can be restored in case they happen to get replaced with
+   broken or alternative objects.
+
+   .. versionadded:: 3.10
 
 .. function:: get_ident()