Refs #23919 -- Removed __traceback__ setting needed for Python 2.

Partially reverted refs #25761 and refs #16245.

django/dispatch/dispatcher.py

@@ -1,4 +1,3 @@
-import sys
 import threading
 import weakref
 
@@ -200,8 +199,7 @@ class Signal:
 
         If any receiver raises an error (specifically any subclass of
         Exception), the error instance is returned as the result for that
-        receiver. The traceback is always attached to the error at
-        ``__traceback__``.
+        receiver.
         """
         if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
             return []
@@ -213,8 +211,6 @@ class Signal:
             try:
                 response = receiver(signal=self, sender=sender, **named)
             except Exception as err:
-                if not hasattr(err, '__traceback__'):
-                    err.__traceback__ = sys.exc_info()[2]
                 responses.append((receiver, err))
             else:
                 responses.append((receiver, response))