docs: def join_template_name(self, template_name, extension):
          """
          Appends an extension to a template_name or list of template_names.
          """
          if template_name is None:
              return None
          if isinstance(template_name, (list, tuple)):
              return tuple('.'.join([n, extension]) for n in template_name)
          if isinstance(template_name, str_types):
              return '.'.join([template_name, extension])
          raise AssertionError('template_name not of correct type: %r' % type(template_name))