Ditching iris

[mine-cetinkaya-rundel]

The first sheet in datasets.xlsx is iris. Would be good to replace that with another dataset, or just remove it and use the remaining sheets.

[jennybc]

Such a swap (e.g. penguins for iris) will affect readxl documentation and perhaps tests, at the very least.

And it's possible that other packages use that file.

So it's a feasible change but not as small as it first looks.