Fixes test_exceptions.py for Python3

- XML is parsed as bytes
- The XML returned from to_string is decoded from a byte string to a
  text string for the comparison.
- Exception.message no longer exists in Python 3

bp python3

Change-Id: I3305433e52c0def422d9f04a4d9f04c6f017d10e
Merge "Fixes test_exceptions.py for Python3"