Mark, I'm not clear if the line number can still be None in some circumstances. 

With this code (from site.py in the stdlib):

545: def execsitecustomize():
546:     """Run custom site specific code, if available."""
547:     try:
548:         try:
549:             import sitecustomize
550:         except ImportError as exc:
551:             if exc.name == 'sitecustomize':
552:                 pass
553:             else:
554:                 raise
555:     except Exception as err:
556:         if sys.flags.verbose:
557:             sys.excepthook(*sys.exc_info())
558:         else:
559:             sys.stderr.write(
560:                 "Error in sitecustomize; set PYTHONVERBOSE for traceback:\n"
561:                 "%s: %s\n" %
562:                 (err.__class__.__name__, err))

I get traces with these events and line numbers:

exception 549 (ModuleNotFoundError("No module named 'sitecustomize'"))
line 550
line 551
line 552
return None

Is this what you expected?