API/BUG: Categorical.is_dtype_equal doesn't compare to Series
xref #16639
```
In [25]: c1 = pd.Categorical(['a', 'b'])

In [26]: c2 = pd.Categorical(['a', 'b'])

In [27]: s = pd.Series(c1)

In [29]: c1.is_dtype_equal(c2)
Out[29]: True

In [30]: c1.is_dtype_equal(s)
Out[30]: False
```

I think `[30]` should be true?