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.

Lib/threading.py

@@ -1200,6 +1200,10 @@ except ImportError:
         stderr.flush()
 
 
+# Original value of threading.excepthook
+__excepthook__ = excepthook
+
+
 def _make_invoke_excepthook():
     # Create a local namespace to ensure that variables remain alive
     # when _invoke_excepthook() is called, even if it is called late during