py3: add alternate output on Python 3
This is basically the same deal as D5806. Python 3's exception
printing output is different.
We had to tweak the regular expression to match appropriately,
hence the added line of output.
Differential Revision: https://phab.mercurial-scm.org/D5807
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