ERR: missing stacklevel in to_dict UserWarning
#### Code Sample, a copy-pastable example if possible

```python
In [4]: df = pd.DataFrame([[1,2,3]], columns=['a', 'a', 'b'])

In [5]: df
Out[5]:
   a  a  b
0  1  2  3

In [6]: df.to_dict()
C:\<snip>\pandas\core\frame.py:891:
UserWarning: DataFrame columns are not unique, some columns will be omitted.
  "columns will be omitted.", UserWarning)
Out[6]: {'a': {0: 2}, 'b': {0: 3}}
```

#### Problem description

warning should specific stacklevel to warn at calling site, xref #9584
