REGR: Buffer dtype mismatch, expected 'Python object' but got a string
- [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
# Your code here
>>> import pandas as pd
>>> df = pd.DataFrame(['o',"o'doul's",'oad','oaf','oafish'])
>>> df = df.astype('|S')
>>> df
             0
0         b'o'
1  b"o'doul's"
2       b'oad'
3       b'oaf'
4    b'oafish'
>>> import numpy as np
>>> df.replace({None:np.nan})


```

#### Problem description

This was working in 1.1.5

```
    df[c].replace({None: np.NaN}, inplace=True)
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/series.py", line 4479, in replace
    return super().replace(
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/generic.py", line 6840, in replace
    return self.replace(
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/series.py", line 4479, in replace
    return super().replace(
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/generic.py", line 6889, in replace
    new_data = self._mgr.replace_list(
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/internals/managers.py", line 664, in replace_list
    bm = self.apply(
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/internals/managers.py", line 427, in apply
    applied = getattr(b, f)(**kwargs)
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/internals/blocks.py", line 910, in _replace_list
    [b.convert(numeric=False, copy=True) for b in result]
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/internals/blocks.py", line 910, in <listcomp>
    [b.convert(numeric=False, copy=True) for b in result]
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/internals/blocks.py", line 2558, in convert
    values = f(None, self.values.ravel(), None)
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/internals/blocks.py", line 2542, in f
    values = soft_convert_objects(
  File "/home/user/anaconda3/envs/uniqueness/lib/python3.8/site-packages/pandas/core/dtypes/cast.py", line 1139, in soft_convert_objects
    values = lib.maybe_convert_objects(values, convert_datetime=True)
  File "pandas/_libs/lib.pyx", line 2125, in pandas._libs.lib.maybe_convert_objects
ValueError: Buffer dtype mismatch, expected 'Python object' but got a string
```

#### Expected Output

Nones are replaced with nans

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

<details>
INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-128-generic
Version          : #131-Ubuntu SMP Wed Dec 9 06:57:35 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.1.5
numpy            : 1.19.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.4
setuptools       : 50.3.2
Cython           : 3.0a6
pytest           : 6.1.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.2
html5lib         : None
pymysql          : None
psycopg2         : 2.8.5 (dt dec pq3 ext lo64)
jinja2           : 3.0.0a1
IPython          : 7.17.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.20
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.52.0
</details>



