BUG: np.argwhere on pandas series
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numpy/numpy#15555 reports an issue with `np.argwhere` on pandas Series. Reporting here for visibility.

MRE:
```python
>>> import numpy as np
>>> import pandas as pd
>>> s = pd.Series(np.random.randn(5), index=['a', 'b', 'c', 'd', 'e'])
>>> np.argwhere(s < 0)
```
which, with `numpy.__version__ ==1.20.0.dev0+046a736`  gives:
**pd.__version__ == 0.25.3:**
```
FutureWarning: Series.nonzero() is deprecated and will be removed in a future version.Use Series.to_numpy().nonzero() instead
array([[3]])
```
**pd.__version__ == 1.0.5:**
```
ValueError: Length of passed values is 1, index implies 5.
```
