Since 0.18, Element.traverse() returns an iterator instead of
intermediate object.  As a result, the return value is always considered
as truthy value.  And it becomes fragile when the caller modifies the
doctree on the loop.

Fixes sphinx-doc/sphinx#9564.

Add a new config variable: python_use_unqualified_type_names.  If enabled,
it goes to suppress the module name of the python reference if it can be
resolved.

.. note::

   $ find sphinx tests LICENSE doc/conf.py -type f -exec sed -i '' -e 's/2007\-20../2007-2021/' {} \;
   $ git co sphinx/locale/**/*.js sphinx/templates/epub3/mimetype