AttributeError: 'NoneType' object has no attribute 'replace'
#### Code Sample, a copy-pastable example if possible

```python
# Your code here

```
#### Problem description
when I  uprade the version 0.23.0,just raise this error 



#### Expected Output
Traceback (most recent call last):
  File "/Users/funny/Documents/AIML/Asywaimai.py", line 1, in <module>
    import pandas as pd
  File "/usr/local/lib/python3.6/site-packages/pandas/__init__.py", line 42, in <module>
    from pandas.core.api import *
  File "/usr/local/lib/python3.6/site-packages/pandas/core/api.py", line 10, in <module>
    from pandas.core.groupby.groupby import Grouper
  File "/usr/local/lib/python3.6/site-packages/pandas/core/groupby/__init__.py", line 2, in <module>
    from pandas.core.groupby.groupby import (
  File "/usr/local/lib/python3.6/site-packages/pandas/core/groupby/groupby.py", line 46, in <module>
    from pandas.core.index import (Index, MultiIndex,
  File "/usr/local/lib/python3.6/site-packages/pandas/core/index.py", line 2, in <module>
    from pandas.core.indexes.api import *
  File "/usr/local/lib/python3.6/site-packages/pandas/core/indexes/api.py", line 4, in <module>
    from pandas.core.indexes.base import (Index,
  File "/usr/local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 7, in <module>
    from pandas._libs import (lib, index as libindex, tslib as libts,
  File "pandas/_libs/index.pyx", line 28, in init pandas._libs.index
  File "pandas/_libs/tslibs/period.pyx", line 59, in init pandas._libs.tslibs.period
  File "/usr/local/lib/python3.6/site-packages/pandas/tseries/offsets.py", line 1092, in <module>
    class CustomBusinessMonthEnd(_CustomBusinessMonth):
  File "/usr/local/lib/python3.6/site-packages/pandas/tseries/offsets.py", line 1093, in CustomBusinessMonthEnd
    __doc__ = _CustomBusinessMonth.__doc__.replace('[BEGIN/END]', 'end')
AttributeError: 'NoneType' object has no attribute 'replace'
#### Output of ``pd.show_versions()``
0.23.0
<details>

[paste the output of ``pd.show_versions()`` here below this line]
```pd.show_version()```
</details>
