Required Sphinx extension setup function.

def setup(app):
    ''' Required Sphinx extension setup function. '''
    app.add_node(
        collapsible_code_block,
        html=(
            html_visit_collapsible_code_block,
            html_depart_collapsible_code_block
        )
    )
    app.add_directive('collapsible-code-block', CollapsibleCodeBlock)