BUG: replace method with regex=True does not work for byte 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.

---

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

```python
import pandas as pd
s = pd.Series([b'abc'])
# This will replace "abc" with "123", output:
#
# 0    b'123'
# dtype: object
#
print(s.replace(b'abc', b'123'))
# But with regex=True, it doesn't work, output:
#
# 0    b'abc'
# dtype: object
#
print(s.replace(b'abc', b'123', regex=True))
```

#### Problem description

It worked in pandas 0.25. It also works when the data is unicode string instead of byte string.

I see a discussion about `regex` parameter in #33302 and one suggestion is to use `str.replace` instead. But that doesn't work for byte string:

```ipython
In [2]: s = pd.Series([b'abc'])

In [3]: s.str.replace(b'abc', b'123')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-da8a12a3193b> in <module>
----> 1 s.str.replace(b'abc', b'123')

/data/wenjun/miniconda3/envs/test_pd/lib/python3.7/site-packages/pandas/core/strings.py in wrapper(self, *args, **kwargs)
   1951                     f"inferred dtype '{self._inferred_dtype}'."
   1952                 )
-> 1953                 raise TypeError(msg)
   1954             return func(self, *args, **kwargs)
   1955

TypeError: Cannot use .str.replace with values of inferred dtype 'bytes'.
```

So it seems my only workaround is to decode the byte string into unicode string before calling the replace. If there are better ways to do it I'm happy to learn. Thanks.

#### Expected Output

```bash
0    b'123'
dtype: object
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-108-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.18.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.3.1.post20200622
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          : 7.15.0
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          : None
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>
