Merge lp:~cjwatson/storm/py3-traceback-reference-cycles into lp:storm

Avoid traceback reference cycles when wrapping exceptions.

`wrap_exceptions` stored the original exception's traceback in a local variable, but the traceback in turn contains a reference to that frame, creating a reference cycle. The situation is worse on Python 3, because the original exception also has the associated traceback as a `__traceback__` attribute, producing more complicated reference cycles. Delete these local variables before returning to avoid leaking memory over time.