BUG: convert_dtypes incorrectly converts byte strings to strings in 1.3+
#### Code Sample, a copy-pastable example

This test will pass on pandas 1.2.5 but fail on 1.3 onwards (checked 1.3, 1.3.2). 
```python
def test_convert_dtypes_on_byte_string():
    byte_str = b'binary-string'
    dataframe = pd.DataFrame(
        data={
            "data": byte_str,
        },
        index=[0]
    )
    converted_dtypes = dataframe.convert_dtypes()
    assert converted_dtypes['data'][0].decode('ascii') == "binary-string"
```

#### Problem description

Error on 1.3 onwards:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 10, in test_convert_dtypes_on_byte_string
AttributeError: 'str' object has no attribute 'decode'
```

The issue is that converted_dtypes() will convert the `data` column values to the string `b'binary-data'`, almost like it has had `str(val)` called on it. On 1.2.5 it is left correctly as a byte array.
 
#### Expected Output

Test should pass. Byte strings should not be converted to string.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 5f648bf1706dd75a9ca0d29f26eadfbb595fe52b
python           : 3.8.11.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.6.0
Version          : Darwin Kernel Version 20.6.0: Wed Jun 23 00:26:31 PDT 2021; root:xnu-7195.141.2~5/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.3.2
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.1.3
setuptools       : 57.4.0
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.9.1 (dt dec pq3 ext lo64)
jinja2           : 3.0.1
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : 0.7.0
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : 2021.07.0
scipy            : None
sqlalchemy       : 1.4.23
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
