implement chained exception tracebacks

patch from Antoine Pitrou #3112
diff --git a/Misc/NEWS b/Misc/NEWS
index e4178c0..a6ecc0e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,8 @@
 
 - Issue #3236: Return small longs from PyLong_FromString.
 
+- Exception tracebacks now support exception chaining.
+
 Library
 -------
 
@@ -35,6 +37,8 @@
 - All the u* variant functions and methods in gettext have been renamed to their
   none u* siblings.
 
+- The traceback module has been expanded to handle chained exceptions.
+
 C API
 -----