[FIX] P3: raise exception with existing traceback

In Python 2, to raise a new exception but reuse a traceback requires a
special form of ``raise`` (``raise etype, evalue, tb``).

In Python 3, this is now done via a ``with_traceback`` method on
exception objects.

However this requires a bit of trickery as the former is invalid
syntax in Python 3, hence pycompat bridge created via an exec for
Python 2.