The link provided by @SkyLin is Oneboxing once more:
https://medium.com/swlh/5-free-python-courses-for-beginners-to-learn-online-e1ca90687caf
It looks like Medium made some changes on their end that exposed some shortcomings in how we generate Oneboxes. Those issues have now been addressed on our end and merged in, making Oneboxing more robust in general (and not just for Medium).