Clear the ``sys.last_traceback``, ``sys.last_type`` and ``sys.last_value``
attributes by deleting them instead of setting them to ``None``. This better
matches the behaviour of the Python standard library.

Clear the :py:attr:`sys.last_traceback`, :py:attr:`sys.last_type`
and :py:attr:`sys.last_value` attributes by deleting them instead
of setting them to ``None``. This better matches the behaviour of
the Python standard library.

Use proper reST attribute markup.
Use proper reST attribute markup.
