BUG: DatetimeIndex has become unhashable in 1.3.1?
- [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.

---

Hi,
Following code below was working with pandas 1.2.5.
It raises now an error with pandas 1.3.1.

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

```python
import pandas as pd
import random

# Right data
ts_open = pd.DatetimeIndex([
                           pd.Timestamp('2021/01/01 00:37'),
                           pd.Timestamp('2021/01/01 00:40'),
                           pd.Timestamp('2021/01/01 01:00'),
                           pd.Timestamp('2021/01/01 03:45'),
                           pd.Timestamp('2021/01/01 03:59'),
                           pd.Timestamp('2021/01/01 05:20')])
length = len(ts_open)
random.seed(1)
volume = random.sample(range(1, length+1), length)
df_smpl = pd.DataFrame({'volume' : volume, 'ts_open': ts_open})

# Left data
ts_full = pd.date_range(start=pd.Timestamp('2021/01/01 00:00'),
                        periods=7, freq='1h')
ts_event = ts_full[:-1]
length_ipr = len(ts_event)
random.seed(2)
volume = random.sample(range(20, length_ipr+20), length_ipr)
df_ipr = pd.DataFrame({'volume' : volume, 'timestamp': ts_event})

at = ts_full[1:]
df_smpl.index = df_smpl['ts_open']

# Test
df_full = pd.merge_asof(df_ipr, df_smpl,
                        left_on=at, right_index=True,
                        allow_exact_matches=False, direction='backward')
```

Error message:
```python
Traceback (most recent call last):

  File "<ipython-input-152-b5845a330a4b>", line 30, in <module>
    df_full = pd.merge_asof(df_ipr, df_smpl,

  File "/home/yoh/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py", line 581, in merge_asof
    op = _AsOfMerge(

  File "/home/yoh/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py", line 1736, in __init__
    _OrderedMerge.__init__(

  File "/home/yoh/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py", line 1619, in __init__
    _MergeOperation.__init__(

  File "/home/yoh/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py", line 682, in __init__
    self._validate_specification()

  File "/home/yoh/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py", line 1782, in _validate_specification
    if left_on in self.left.columns

  File "/home/yoh/anaconda3/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 4572, in __contains__
    hash(key)

TypeError: unhashable type: 'DatetimeIndex'
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
python           : 3.8.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-63-generic
Version          : #71~20.04.1-Ubuntu SMP Thu Jul 15 17:46:08 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.1
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.1.3
setuptools       : 52.0.0.post20210125
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : None
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.22.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : 0.7.0
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.2
sqlalchemy       : None
tables           : None
tabulate         : 0.8.9
xarray           : 0.19.0
xlrd             : None
xlwt             : None
numba            : 0.53.1

</details>
