BUG: Slicing on Datetimeindex fails
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np

df = pd.DataFrame(
    {'col2': [i for i in range(4)]}, 
    index=pd.DatetimeIndex(["11.01.2011 22:00", "11.01.2011 23:00", "12.01.2011 00:00", "2011-01-13 00:00"]))
df["2011-01-01": "2011-11-01"]

```

Error message:

```
  File "...\pandas\core\frame.py", line 3438, in __getitem__
    return self._slice(indexer, axis=0)
  File "...\pandas\core\generic.py", line 3826, in _slice
    assert isinstance(slobj, slice), type(slobj)
AssertionError: <class 'numpy.ndarray'>
```

#### Problem description

The execution of `df["2011-01-01": "2011-11-01"]` fails with the following exception: `AssertionError: <class 'numpy.ndarray'>`.  So the first problem is that the error message is not helping to solve the issue. Moreover, I observed that at least in Pandas 1.0.0 the code snippet is executed successfully. Additionally, if the index is sorted the error also does not occur: `df.sort_index()["2011-01-01": "2011-11-01"]`

Note a similar closed issue exists: #35509. With the current version the code snippet from that issue works. However, mine fails. 

#### Expected Output
                          col2
    2011-01-13 00:00:00     3
    2011-11-01 22:00:00     0
    2011-11-01 23:00:00     1

or helpful error message

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 5f648bf1706dd75a9ca0d29f26eadfbb595fe52b
python           : 3.8.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : de_DE.cp1252
pandas           : 1.3.2
numpy            : 1.18.4
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 53.1.0
Cython           : None
pytest           : 6.2.2
hypothesis       : None
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.20.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : 0.18.2
xlrd             : None
xlwt             : None
numba            : None
</details>
