MAINT: Change all pandas links to use HTTPS
Also update the link to pandas' `whatsnew` page
> any way to check these all work?

This is a simple change from HTTP to HTTPS for all of the `pandas.pydata.org` links. `pandas.pydata.org` supports HTTPS across the board.

The question of whether these links work is actually not relevant here.  If they don't work, they wouldn't have worked on HTTP.