IPython directive: correctly clear cout

IPython/sphinxext/ipython_directive.py

@@ -585,7 +585,7 b' class EmbeddedSphinxShell(object):'
                 if self.warning_is_error:
                     raise RuntimeError('Non Expected warning in `{}` line {}'.format(filename, lineno))
 
-        self.cout.truncate(0)
+        self.clear_cout()
         return (ret, input_lines, processed_output,
                 is_doctest, decorator, image_file, image_directive)