Accept CategoricalDtype in read_csv
```python
import pandas as pd
from io import StringIO
from pandas.api.types import CategoricalDtype

data = 'col1,col2,col3\na,b,1\na,b,2\nc,d,3'

dtype = CategoricalDtype(['d', 'c', 'b', 'a'], ordered=True)
pd.read_csv(StringIO(data), dtype={'col1': dtype}).dtypes
```


This is for after https://github.com/pandas-dev/pandas/pull/16015

cc @chris-b1 