BUG: Possible Regression in DatetimeIndex Slicing
- [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
>>> df = pd.DataFrame(
...     {'col1': ['a', 'b', 'c'], 'col2': [1, 2, 3]}, 
...     index=pd.to_datetime(['2020-08-01', '2020-07-02', '2020-08-05'])
... )
>>> df['2020-08']
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-82-c8d0440364af> in <module>
----> 1 df['2020-08']

~/book_env/lib/python3.7/site-packages/pandas/core/frame.py in __getitem__(self, key)
   2880             # either we have a slice or we have a string that can be converted
   2881             #  to a slice for partial-string date indexing
-> 2882             return self._slice(indexer, axis=0)
   2883 
   2884         # Do we have a (boolean) DataFrame?

~/book_env/lib/python3.7/site-packages/pandas/core/generic.py in _slice(self, slobj, axis)
   3546         Slicing with this method is *always* positional.
   3547         """
-> 3548         assert isinstance(slobj, slice), type(slobj)
   3549         axis = self._get_block_manager_axis(axis)
   3550         result = self._constructor(self._mgr.get_slice(slobj, axis=axis))

AssertionError: <class 'numpy.ndarray'>
```

You now have to run `sort_index()` before slicing:
```python
>>> df.sort_index()['2020-08']
           col1  col2
2020-08-01    a     1
2020-08-05    c     3
```

#### Problem description

Before 1.1.0, you didn't have to sort the index before slicing. Now, you have to run `sort_index()` and the error message is not helpful at all.

#### Expected Output
```python
>>> df['2020-08']
           col1  col2
2020-08-01    a     1
2020-08-05    c     3
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.40-04224-g891a6cce2d44
Version          : #1 SMP PREEMPT Tue Jun 23 20:21:29 PDT 2020
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.2
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: 0.9.0
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.0
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.18
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
