BUG: indexing regression with datetime index
Sample code:
```python
import pandas as pd

idx = pd.date_range('2008', '2009')
idx.to_series()['2008']
```

in 0.25.3 we get
````
2008-01-01   2008-01-01
2008-01-02   2008-01-02
2008-01-03   2008-01-03
2008-01-04   2008-01-04
2008-01-05   2008-01-05
                ...    
2008-12-27   2008-12-27
2008-12-28   2008-12-28
2008-12-29   2008-12-29
2008-12-30   2008-12-30
2008-12-31   2008-12-31
Freq: D, Length: 366, dtype: datetime64[ns]
````

in >= 1.0.0 (up to 1.0.4 at least) we get
```
2008   2008-01-01
2008   2008-01-02
2008   2008-01-03
2008   2008-01-04
2008   2008-01-05
          ...    
2008   2008-12-27
2008   2008-12-28
2008   2008-12-29
2008   2008-12-30
2008   2008-12-31
Length: 366, dtype: datetime64[ns]
```

The index has dtype obj

Variantations like
```
idx.to_series().astype(str)['2008']
idx.to_series().loc['2008']
```
look unaffected
