BUG: Series.argmax() fails with np.inf
closes #13595

This PR changes the behavior of two functions in the `nanops` module to address #13595, `nanargmax` and `nanargmin`. Previously, these two functions used a flag on the `_get_values` helper that caused it to 1) always mask out and ignore infinite values in the input, and 2) attempt to coerce the input to float.

It's worth noting that because of 2), this changes the way `nanargmax` and `nanargmin`, and consequently `Series.idxmax` etc, behave with string data. Previously,

```python
data = ['foo', 'bar', 'baz']

pd.Series(data).idxmax() # --> ValueError

pd.DataFrame({'A': data}).idxmax() # --> ValueError

pd.DataFrame({'grp': [1, 1, 2], 'A': data}).groupby('grp').idxmax() # --> an empty data frame
```

After the PR,

```python
In [1]: import pandas as pd

In [2]: data = ['foo', 'bar', 'baz']

In [3]: pd.Series(data).idxmax()
Out[3]: 0

In [4]: pd.DataFrame({'A': data}).idxmax()
Out[4]:
A    0
dtype: int64

In [5]: pd.DataFrame({'grp': [1, 1, 2], 'A': data}).groupby('grp').idxmax()
Out[5]:
     A
grp
1    0
2    2
```

This is consistent with the behavior of `numpy.argmax` etc with string data, and might be more consistent with user expectations. It did mean that I needed to update one of the `DataFrame.groupby` test cases, but otherwise appears to cause no breakage.

Since this might be a bigger change that was expected for fixing the issue, I'm posting it as WIP.

 - [x] closes #13595
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry
