code: def _next_slide_partname(self):
          
          sldIdLst = self._element.get_or_add_sldIdLst()
          partname_str = '/ppt/slides/slide%d.xml' % (len(sldIdLst)+1)
          return PackURI(partname_str)
docs: Return |PackURI| instance containing the partname for a slide to be
          appended to this slide collection, e.g. ``/ppt/slides/slide9.xml``
          for a slide collection containing 8 slides.