SettingWithCopyWaraning when modifying a column that is the aggregation of 2 others
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({'c1': [1, 2, 3], 'c2': [3, 4, 5]})
df['c3'] = df['c1'] / df['c2']
df['c3'][0] = 0
```

I haven't tested with other arithmetic operators but I'm guessing it would happen for them as well.