[BUG] loc indexing with length-1 Categorical not possible

[wence-]

Describe the bug

import cudf
import pandas as pd
f = pd.Series([1, 2, 3], index=pd.CategoricalIndex([0, 1, 2]))
f.loc[pd.Categorical([1])] # => second row.
cf = cudf.from_pandas(f)
cf.loc[pd.Categorical([1])] # => KeyErrror

I think this is similar to #13013 in that length-one categoricals are treated as scalars in some cases.

Expected behavior

Probably this lookup should work.