BUG: offsets are now unhashable
- [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.

---
As discussed on gitter, this was apparently an unindended consequence of https://github.com/pandas-dev/pandas/pull/34227

![image](https://user-images.githubusercontent.com/881019/96518983-56d04a80-12af-11eb-951d-89b02badf9df.png)
https://gitter.im/pydata/pandas?at=5f8d8a106c8d484be294872f

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

```python
>>> import pandas as pd
>>> hash(pd.offsets.Day())
Traceback (most recent call last):
  File "C:\Users\dhirschf\envs\dev\lib\site-packages\IPython\core\interactiveshell.py", line 3417, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-46-d78c0d3ee4ad>", line 1, in <module>
    hash(pd.offsets.Day())
TypeError: unhashable type: 'pandas._libs.tslibs.offsets.Day'
```

#### Problem description

My code relied on the hashability of offsets and so this broke my code and has so far prevented me upgrading to the latest `pandas`

#### Expected Output

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

<details>

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.7.7.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.17763
machine          : AMD64
processor        : Intel64 Family 6 Model 58 Stepping 0, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 1.1.3
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.4
setuptools       : 50.3.0.post20201006
Cython           : 0.29.21
pytest           : 6.1.1
hypothesis       : 5.37.3
sphinx           : 3.2.1
blosc            : 1.9.2
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : 4.6.0
html5lib         : 1.1
pymysql          : 0.10.1
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 0.8.4
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : 1.0.1
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.20
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.16.1
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.51.2

```
</details>
