Open a file for reading.

        Use as

        .. code-block:: python

            with app.open_resouce(path) as file_:
                file_.read()

def open_resource(self, path: FilePath, mode: str='rb') -> IO[AnyStr]:
        """Open a file for reading.

        Use as

        .. code-block:: python

            with app.open_resouce(path) as file_:
                file_.read()
        """
        if mode not in {'r', 'rb'}:
            raise ValueError('Files can only be opened for reading')
        return open(self.root_path / file_path_to_path(path), mode)