TypeError on argmax of object dtype (change from 0.20.3)

```python
>>> import pandas as pd
>>> pd.Series([0, 0], dtype='object').argmax()
```

---

I was doing `action = state_action.idxmax()` where state_action was of type 'pandas.core.series.Series'. When I run in 0.21.0, it gives the following error:

>   File "/usr/local/lib/python3.5/dist-packages/pandas/core/series.py", line 1357, in idxmax
>    i = nanops.nanargmax(_values_from_object(self), skipna=skipna)
>  File "/usr/local/lib/python3.5/dist-packages/pandas/core/nanops.py", line 74, in _f
>    raise TypeError(msg.format(name=f.__name__.replace('nan', '')))
> TypeError: reduction operation 'argmax' not allowed for this dtype

However, when I downgraded to pandas 0.20.3, it worked just fine. You might wanna look into this. :)

