Allow Keyword Aggregation in DataFrame.agg and Series.agg
Followup to https://github.com/pandas-dev/pandas/pull/26399

```python
In [2]: df = pd.DataFrame({"A": [1, 2, 1, 2], "B": [1, 2, 3, 4]})

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

In [4]: df.agg(foo=("B", "sum"))
```

Expected Output

```python
In [13]: df.agg({"B": {"foo": "sum"}})
/Users/taugspurger/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/frame.py:6284: FutureWarning: using a dict with renaming is deprecated and will be removed in a future version
  result, how = self._aggregate(func, axis=axis, *args, **kwargs)
Out[13]:
      B
foo  10
```


without the warning. Similar for `Series.agg`

```python
In [16]: df.B.agg({"foo": "sum"})  # allow foo="sum"
Out[16]:
foo    10
Name: B, dtype: int64
```
