Add a config item to this section.

def add_item(self, alias, item):
        """
        Add a config item to this section.
        """
        if not isinstance(alias, six.string_types):
            raise TypeError('Item name must be a string, got a {!r}'.format(type(alias)))
        item = copy.deepcopy(item)
        if item.name is not_set:
            item.name = alias

        if self.settings.str_path_separator in item.name:
            raise ValueError(
                'Item name must not contain str_path_separator which is configured for this Config -- {!r} -- '
                'but {!r} does.'.format(self.settings.str_path_separator, item)
            )

        self._tree[item.name] = item

        if item.name != alias:
            if self.settings.str_path_separator in alias:
                raise ValueError(
                    'Item alias must not contain str_path_separator which is configured for this Config -- {!r} --'
                    'but {!r} used for {!r} does.'.format(self.settings.str_path_separator, alias, item)
                )
            self._tree[alias] = item

        item._section = self

        self.dispatch_event(self.hooks.item_added_to_section, alias=alias, section=self, subject=item)