Suppress exception chaining when PEP 3134 is merged

[atagar]

Migrated from ticket 16348.

Python3 added exception chaining. Under python2 you only get the useful stacktrace but with python3 you get the internal one too.

At present there isn't a method for suppressing this. ​There's a proposal for adding suppression but it isn't yet merged, and even after it is this will be an ongoing annoyance for python 3.x before the addition.