Issue #7376: When called with no arguments doctest was running a

Issue #7376: When called with no arguments doctest was running a
self-test.  Because of a change to the way tracebacks are printed,
this self-test was failing.  The test is run (and passes) during normal
regression testing.  So instead of running the failing self-test this
patch makes doctest emit a usage message.  This is better behavior anyway
since passing in arguments is the real reason to run doctest as a command.

Bug discovery and initial patch by Florent Xicluna.