py3: add extra traceback line present on Python 3.8

I'm not sure why Python 3.8 is outputting this line. It
appears to be a change in behavior of formatting tracebacks on
Python 3.8. So let's add it to expected output.

With this change, test-hook.t now passes on Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D7946

tests/test-hook.t

@@ -988,6 +988,7 b' test python hooks'
   ModuleNotFoundError: No module named 'hgext_syntaxerror' (py36 !)
   Traceback (most recent call last): (py3 !)
   HookLoadError: preoutgoing.syntaxerror hook is invalid: import of "syntaxerror" failed (no-py3 !)
+      raise error.HookLoadError( (py38 !)
   mercurial.error.HookLoadError: preoutgoing.syntaxerror hook is invalid: import of "syntaxerror" failed (py3 !)
   abort: preoutgoing.syntaxerror hook is invalid: import of "syntaxerror" failed
 
@@ -1161,6 +1162,7 b' make sure --traceback works on hook impo'
   ModuleNotFoundError: No module named 'hgext_importfail' (py36 !)
   Traceback (most recent call last):
   HookLoadError: precommit.importfail hook is invalid: import of "importfail" failed (no-py3 !)
+      raise error.HookLoadError( (py38 !)
   mercurial.error.HookLoadError: precommit.importfail hook is invalid: import of "importfail" failed (py3 !)
   abort: precommit.importfail hook is invalid: import of "importfail" failed