Pandas updates and deprecations

[doctor-phil]

Pandas is constantly updating and making big changes and deprecating methods; some of which are core to certain lectures. It's getting harder to keep that pinned version <2.0, since the student's environments are more up to date.

I am worried that eventually upgrading pandas will have huge impacts on some of these notes. Especially the lecture on reshaping.

Collecting relevant substantial changes here so I can go back and try to update it for next term.

• [ ] stack and unstack changed behavior somehow
• [ ] issue with downcasting on pd.DataFrame.fillna?
• [ ] possible changes to bfill and ffill

add more here as they arise