BUG: loc raising KeyError for string slices in list-like indexer and DatetimeIndex
- [x] closes #27180
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I looked into this and added an implementation for list-like indexers containing slices and DatetimeIndexes. If we want to support this we probably have to do something like this.
There are a couple of issues here, first of which is that it isn't obvious that we actually should support this.  It does improve internal consistency, but adds a lot of complexity.

Even if we do decide to move forward with this, id want to start with making get_indexer behave like a vectorized get_loc.

(ideally id like to get rid of of the `FooIndex._convert_bar_indexer` methods altogether) > There are a couple of issues here, first of which is that it isn't obvious that we actually should support this. It does improve internal consistency, but adds a lot of complexity.
> 
> Even if we do decide to move forward with this, id want to start with making get_indexer behave like a vectorized get_loc.
> 
> (ideally id like to get rid of of the `FooIndex._convert_bar_indexer` methods altogether)

+1 here. yeah partial date indexing has only very limited support now, we don't wish to expand this, its mainly useful for interactive use. 

unless other objections going to close the original issue.

thanks @phofl for the effort here. No worries, I was not sure either if this would be desirable. But wanted to share a possible solution before discussing this.  did we settle on closing this? Think so? closing, though we should have a test and good error message if someone attempst this. @phofl want to repurpose this? Yep will do  Currently 

```
index = pd.date_range('2001-01-01', periods=100)


s = pd.Series(1, index=index)
s.loc[['2001-01']]
```

returns
```
2001-01-01    1
dtype: int64
```

which is obviously wrong. I would change this to raise if this is attempted. > `s.loc[['2001-01']]`

I'm not sure how avoidable this is given that `Timestamp("2001-01")` returns 2001-01-01, but if you've got an improvement in mind, go for it Yep, you are probably right. Have not found a way to avoid this. Closing this pr.