write `content` to a temporary XXX`suffix` file and return the filename.
       It is user's responsibility to delete when done.

def _write_temp(content, suffix):
    '''write `content` to a temporary XXX`suffix` file and return the filename.
       It is user's responsibility to delete when done.'''
    with NamedTemporaryFile(mode='w', suffix=suffix, delete=False) as out:
        out.write(content)
        return out.name