Fix Python 2's traceback.format_exception, traceback.print_exception (#1512)
Fix Python 2's traceback.format_exception, traceback.print_exception (#1512)

E.g. it's legal to call traceback.format_exception(None, None, None). In
particular, this change makes the following idiom type-check:
  import traceback
  import sys
  exc_type, exc_value, exc_traceback = sys.exc_info()
  traceback.format_exception(exc_type, exc_value, exc_traceback)
euresti/typeshed,li-dan/typeshed