Undo the pin of `scipy <= 1.4.1` after scipy 1.5.2 is out.

[ev-br]

Had to add in #183 for otherwise scipy==1.5.1 was failing to from scipy.stats import levene.
The error was somewhere in scipy internals, it was failing to import something about highspy. So hopefully it'll resolve itself once 1.5.2 is available.