BUG: KeyError on MultiIndex when filtering by only two indexes
- [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.

#### Code

```python
import pandas as pd
df = pd.DataFrame({'col1':[1,2,1,2],'col2':['a','a','b','b'],'col3':['US','US','US','BR'],'values':[1.0,2.0,3.0,4.5]})
df.set_index(['col1','col2','col3'],inplace = True)
df.loc[([1],['a'])]
```
```python-traceback
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1762, in __getitem__
    return self._getitem_tuple(key)
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1272, in _getitem_tuple
    return self._getitem_lowerdim(tup)
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1373, in _getitem_lowerdim
    return self._getitem_nested_tuple(tup)
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1453, in _getitem_nested_tuple
    obj = getattr(obj, self.name)._getitem_axis(key, axis=axis)
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1954, in _getitem_axis
    return self._getitem_iterable(key, axis=axis)
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1595, in _getitem_iterable
    keyarr, indexer = self._get_listlike_indexer(key, axis, raise_missing=False)
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1553, in _get_listlike_indexer
    keyarr, indexer, o._get_axis_number(axis), raise_missing=raise_missing
  File "/home/fabio.rangel/projects/audience_topic_viewership/.direnv/python-3.7.7/lib/python3.7/site-packages/pandas/core/indexing.py", line 1640, in _validate_read_indexer
    raise KeyError(f"None of [{key}] are in the [{axis_name}]")
KeyError: "None of [Index(['a'], dtype='object')] are in the [columns]"
```
#### Problem description

The problem occurs only when filtering using MultiIndex with a two-sized tuple. When using one, three or more it does not occurs. I tested using up to four indexes. 

#### Expected Output
```sh
                values
col1 col2 col3        
1    a    US       1.0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-174-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : pt_BR.UTF-8
LOCALE           : pt_BR.UTF-8

pandas           : 1.0.5
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.3.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.16.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.17.1
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
