py3: add Python 3 output for test-flagprocessor.t
The exception is being generated from ui.traceback(), which
simply calls traceback.format_*() to format exceptions. Since
the output from the standard library has changed, there is
seemingly not much we can do about it. So this commit adds
conditional output depending on the Python version.
I'm not thrilled about b'' appearing in user-facing output.
Can we do something better here?
Differential Revision: https://phab.mercurial-scm.org/D5806