REGR: comparison op with dask data structure fails
Using pandas 1.0.5 and latest dask 2020.12.0:

```
In [2]: import dask.dataframe as dd                                                                                                                                                                                

In [3]: df = pd.DataFrame({"x": ["a", "b", "c"] * 100}, dtype="category") 
   ...: ddf = dd.from_pandas(df, npartitions=3)                                                                                                                                                                

In [4]: df.x                                                                                                                                                                                                       
Out[4]: 
0      a
1      b
2      c
3      a
4      b
      ..
295    b
296    c
297    a
298    b
299    c
Name: x, Length: 300, dtype: category
Categories (3, object): [a, b, c]

In [5]: ddf.x                                                                                                                                                                                                      
Out[5]: 
Dask Series Structure:
npartitions=3
0      category[known]
100                ...
200                ...
299                ...
Name: x, dtype: category
Dask Name: getitem, 6 tasks

In [6]: df.x == ddf.x                                                                                                                                                                                              
Out[6]: 
0      True
1      True
2      True
3      True
4      True
       ... 
295    True
296    True
297    True
298    True
299    True
Name: x, Length: 300, dtype: bool

In [9]: (df.x == ddf.x).all()                                                                                                                                                                                      
Out[9]: True
```

But with master (using same dask version), this gives:

```
In [3]: df.x == ddf.x
Out[3]: 
0      False
1      False
2      False
3      False
4      False
       ...  
295    False
296    False
297    False
298    False
299    False
Name: x, Length: 300, dtype: bool
```