Allow Keyword aggregation in SeriesGroupBy
Followup to https://github.com/pandas-dev/pandas/pull/26399

```python
In [1]: df = pd.DataFrame({'A': [1, 1, 1, 2, 2], 
   ...:                      'B': range(5), 
   ...:                      'C': range(5)})                                    

In [2]: df                                     
Out[2]: 
   A  B  C
0  1  0  0
1  1  1  1
2  1  2  2
3  2  3  3
4  2  4  4

In [3]: df.groupby('A').B.agg(foo='count')                                                        
Out[3]: 
   foo
A     
1    3
2    2
```

https://github.com/pandas-dev/pandas/pull/26399#pullrequestreview-241906201

