API: CategoricalDtype str, repr

[TomAugspurger]

I think that str(cat.dtype) should be changed back to always being 'category'. I've seen several places where people use str(thing.dtype) == 'category' as a way to check for Categoricals. Even subtle things like this arrow PR would break.

So instead of

In [6]: str(pd.Categorical([1, 2, 3]).dtype)
Out[6]: 'CategoricalDtype(categories=[1, 2, 3], ordered=False)'

it would be

Out[6]: 'category'

We can leave __repr__ to be unambiguous.

[TomAugspurger]

Double posted with #17782