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

tests/test-flagprocessor.t

@@ -209,11 +209,13 b' Ensure the data got to the server OK'
       _insertflagprocessor(flag, processor, _flagprocessors)
     File "*/mercurial/revlog.py", line *, in _insertflagprocessor (glob)
       raise error.Abort(msg)
-  Abort: cannot register multiple processors on flag '0x8'.
+  mercurial.error.Abort: b"cannot register multiple processors on flag '0x8'." (py3 !)
+  Abort: cannot register multiple processors on flag '0x8'. (no-py3 !)
   *** failed to set up extension duplicate: cannot register multiple processors on flag '0x8'.
   $ hg st 2>&1 | egrep 'cannot register multiple processors|flagprocessorext'
     File "*/tests/flagprocessorext.py", line *, in extsetup (glob)
-  Abort: cannot register multiple processors on flag '0x8'.
+  mercurial.error.Abort: b"cannot register multiple processors on flag '0x8'." (py3 !)
+  Abort: cannot register multiple processors on flag '0x8'. (no-py3 !)
   *** failed to set up extension duplicate: cannot register multiple processors on flag '0x8'.
     File "*/tests/flagprocessorext.py", line *, in b64decode (glob)