Use utf-8 encoding for traceback source with no encoding in py3k

Added a default encoding of "utf-8" when the :class:`.RichTraceback`
object retrieves Python source lines from a Python traceback; as these
are bytes in Python 3 they need to be decoded so that they can be
formatted in the template.

Fixes: #293
Change-Id: I41a5c09422d6500c7cab2423ed14ac951a64e2f4