skip over functions with private names (as indicated by the underscore
naming convention).  The old default created too much of a risk that
user tests were being skipped inadvertently.  Note, this change could
break code in the unlikely case that someone had intentionally put
failing tests in the docstrings of private functions.  The breakage
is easily fixable by specifying the old behavior when calling testmod()
or Tester().  The more likely case is that the silent failure was
unintended and that the user needed to be informed so the test could be
fixed.