Remove expectation for UnicodeDecodeError in a unittest.

It seems that more modern versions of Python (e.g. 2.7.8), doing:

    from __future__ import print_function
    print("SIG=%s" % sig)

with some binary data doesn't actually produce a UnicodeDecodeError,
which means that Python2 is slowly getting less retarded. :D

On the downside, we probably shouldn't have been testing for things
which are Python2.x mis-design issues (and not really our fault, nor a
bug in python-gnupg) anyway.