Query re Deprecation of groupby.agg() with a dictionary when renaming
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
df = pd.DataFrame({'City':['LA', 'NYC', 'NYC', 'LA', 'Chicago', 'NYC'],
       'isFraud':[1, 0, 0, 1, 0, 1]})

df.groupby(df['City'])['isFraud'].agg({'Fraud':sum, 'Non-Fraud': lambda x: len(x)-sum(x), 'Squared': lambda x: (sum(x))**2})
```
#### Problem description

I just learnt using a dictionary for renaming in agg is going to be deprecated in the latest version. My question is what's the alternative to achieve the above, i.e. using multiple lambda functions within agg?
#### Expected Output

```
         Non-Fraud  Fraud  Squared
City                              
Chicago          1      0        0
LA               0      2        4
NYC              2      1        1
```

#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here

</details>
