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

tests/test-worker.t

@@ -83,8 +83,10 b' Known exception should be caught, but pr'
   [255]
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
-  > test 100000.0 abort --traceback 2>&1 | egrep '^(SystemExit|Abort)'
-  Abort: known exception
+  > test 100000.0 abort --traceback 2>&1 | egrep '(SystemExit|Abort)'
+      raise error.Abort(b'known exception')
+  mercurial.error.Abort: b'known exception' (py3 !)
+  Abort: known exception (no-py3 !)
   SystemExit: 255
 
 Traceback must be printed for unknown exceptions