BUG: Problem using to_csv with BytesIO
Dear people of Pandas,

first things first: Thanks for all of your excellent work conceiving and maintaining Pandas. You know who you are.

With kind regards,
Andreas.

---

- [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.

---

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

```python
import pandas as pd
from io import BytesIO

df = pd.DataFrame()
buffer = BytesIO()

df.to_csv(buffer)
```

#### Problem description

The snippet above croaks with
```
TypeError: a bytes-like object is required, not 'str'
```

We expected this to work. However, we will be happy to learn otherwise. We also had a look at #22555 and #35129 which seem to be related but not exactly on the spot.

The background on this is that we are currently in the process of upgrading [Kotori](https://github.com/daq-tools/kotori) to Python 3 (yeah, we are late to the game). However, coming from this, we can confirm it worked when using Pandas 0.18.1 on Python 2 the other day.

The relevant code is
- https://github.com/daq-tools/kotori/blob/0.24.5/kotori/io/protocol/http.py#L8
- https://github.com/daq-tools/kotori/blob/0.24.5/kotori/io/protocol/http.py#L588-L602

Thanks already for looking into this!

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

<details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.6.9.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 17.7.0
Version          : Darwin Kernel Version 17.7.0: Thu Jun 18 21:21:34 PDT 2020; root:xnu-4570.71.82.5~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.1.3
numpy            : 1.19.2
pytz             : 2018.9
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 50.3.0
Cython           : None
pytest           : 4.6.9
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.3.6
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.8
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
