Return the section and key in dot notation format.

def _to_dot_key(cls, section, key=None):
        """ Return the section and key in dot notation format. """
        if key:
            return (NON_ALPHA_NUM.sub('_', section.lower()), NON_ALPHA_NUM.sub('_', key.lower()))
        else:
            return NON_ALPHA_NUM.sub('_', section.lower())