Since traceback.print_exception already uses print statememts, your patch *is* sufficient to trap the remaining stderr exception. Go ahead.

The doctring for report_callback_exception calls it an 'internal function'.  To me, that implies 'ignore this' rather than 'override this'.  I suggest changing the docstring to

"""Report callback exception on sys.stderr.

Applications may want to override this internal function, and should when sys.stderr is None.
"""