API: CategoricalDtype str, repr
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](https://github.com/apache/arrow/pull/1161/files#diff-4b312e1fbf7b4c070f9788be9db33b20R198) 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.
