BUG: String indexing against object dtype may raise AttributeError
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

String indexing may raise `AttributeError`, rather than `KeyError`.

```
df = pd.DataFrame([1], pd.Index([pd.Timestamp('2011-01-01')], dtype=object))
df.index.is_all_dates
# True
df['2011']
# AttributeError: 'Index' object has no attribute '_get_string_slice'
```

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14424?src=pr) is 85.25% (diff: 100%)

> Merging [#14424](https://codecov.io/gh/pandas-dev/pandas/pull/14424?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14424   diff @@
==========================================
  Files           140        140          
  Lines         50631      50632     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43167      43168     +1   
  Misses         7464       7464          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7cad3f1...ae22dce](https://codecov.io/gh/pandas-dev/pandas/compare/7cad3f16bccd1d4702ef9d038b1ee0db33b9bb94...ae22dce62e6d97d79d8786c6a16be7ca230799de?src=pr)
 Small comment, looks good for the rest
 @sinhrks Thanks!
