miss dep: python-sphinx-notfound-page

Description:

Upstream added python-sphinx-notfound-page as python-pallets-sphinx-themes 's dependency in https://github.com/pallets/pallets-sphinx-themes/pull/118

Reported-by: https://archriscv.felixc.at/.status/log.htm?url=logs/python-flask-sqlalchemy/python-flask-sqlalchemy-3.1.1-4.log